I won't argue with you there, although it would be more accurate to say Tera Blast specifically made it broken
Was it really Tera, and not say Speed Boost or Stored Power? Banning Tera would increase the offensive counterplay a bit, but defensive counterplay would still be very limited as it can still run Fairy coverage, not to mention it can still be just as much of a matchup fish with moves like Roost and Substitute.
You're talking about a Pokémon with an absurdly high (effective) attack stat and with a very effective STAB combo, with anything that could remotely try to switch up to it (Zama-C, Magearna) being banned or, in the case of Dondozo, still being exploitable. Obviously it appreciates Tera, but if Lando-I were dropped down here, it would appreciate Tera too.
Volc is probably the second most likely Pokémon to be broken due to Tera, but you can't ignore how all of its "counters" received major nerfs this generation and that Volc was always a bit of a matchup fish anyway.
Annihilape was banned specifically due to its matchup with bulky teams; HO never really had much difficulty facing it. And I don't see how a Tera ban is supposed to help with that matter, at best they would just be relying on a Flame Body proc to deal with it.
You can't just say "Tera made X broken" like it's the most obvious thing in the world as most of these Pokémon weren't tested in a no-Tera environment. Regieleki is the only real exception, since if anything it was nerfed from last gen but coverage against grounds clearly put it over the edge. But for the others, we have Pokémon that at best would just be teetering over the edge of brokenness and at worst would be just as broken as before if not moreso.
So something interesting is that a Teraless OU tournament is currently happening at the moment, I took the liberty of compiling the stats up to now to get some insight on usage percentages as well as win percentages. An interesting thing is that the TD team decided to unban Regieleki, Espathra and Volcarona from this tournament. The stats are below:


Code:
+ ---- + ------------------ + ---- + ------- + ------- +
| Rank | Pokemon | Use | Usage % | Win % |
+ ---- + ------------------ + ---- + ------- + ------- +
| 1 | Kingambit | 24 | 46.15% | 54.17% |
| 2 | Great Tusk | 23 | 44.23% | 60.87% |
| 3 | Gholdengo | 18 | 34.62% | 27.78% |
| 4 | Iron Valiant | 14 | 26.92% | 14.29% |
| 5 | Ting-Lu | 13 | 25.00% | 69.23% |
| 6 | Zapdos | 12 | 23.08% | 58.33% |
| 6 | Walking Wake | 12 | 23.08% | 50.00% |
| 8 | Samurott-Hisui | 11 | 21.15% | 18.18% |
| 9 | Dragapult | 10 | 19.23% | 70.00% |
| 10 | Cinderace | 8 | 15.38% | 25.00% |
| 11 | Iron Moth | 7 | 13.46% | 85.71% |
| 11 | Baxcalibur | 7 | 13.46% | 71.43% |
| 11 | Greninja-* | 7 | 13.46% | 71.43% |
| 11 | Enamorus | 7 | 13.46% | 71.43% |
| 15 | Torkoal | 6 | 11.54% | 66.67% |
| 15 | Moltres | 6 | 11.54% | 50.00% |
| 15 | Slowking-Galar | 6 | 11.54% | 50.00% |
| 15 | Sneasler | 6 | 11.54% | 33.33% |
| 15 | Landorus-Therian | 6 | 11.54% | 16.67% |
| 20 | Toxapex | 5 | 9.62% | 100.00% |
| 20 | Pelipper | 5 | 9.62% | 60.00% |
| 20 | Amoonguss | 5 | 9.62% | 40.00% |
| 20 | Volcarona | 5 | 9.62% | 20.00% |
| 24 | Corviknight | 4 | 7.69% | 75.00% |
| 24 | Dondozo | 4 | 7.69% | 75.00% |
| 24 | Glimmora | 4 | 7.69% | 75.00% |
| 24 | Brute Bonnet | 4 | 7.69% | 75.00% |
| 24 | Basculegion | 4 | 7.69% | 50.00% |
| 24 | Espathra | 4 | 7.69% | 50.00% |
| 24 | Volcanion | 4 | 7.69% | 50.00% |
| 24 | Clodsire | 4 | 7.69% | 50.00% |
| 24 | Hatterene | 4 | 7.69% | 25.00% |
| 33 | Dragonite | 3 | 5.77% | 66.67% |
| 33 | Meowscarada | 3 | 5.77% | 33.33% |
| 33 | Azumarill | 3 | 5.77% | 33.33% |
| 33 | Rotom-Wash | 3 | 5.77% | 0.00% |
| 33 | Sandy Shocks | 3 | 5.77% | 0.00% |
| 38 | Blissey | 2 | 3.85% | 100.00% |
| 38 | Zamazenta-* | 2 | 3.85% | 100.00% |
| 38 | Iron Hands | 2 | 3.85% | 50.00% |
| 38 | Heatran | 2 | 3.85% | 50.00% |
| 38 | Roaring Moon | 2 | 3.85% | 50.00% |
| 38 | Breloom | 2 | 3.85% | 50.00% |
| 38 | Garchomp | 2 | 3.85% | 0.00% |
| 38 | Scream Tail | 2 | 3.85% | 0.00% |
| 38 | Ursaluna | 2 | 3.85% | 0.00% |
| 38 | Scizor | 2 | 3.85% | 0.00% |
| 48 | Tornadus-Therian | 1 | 1.92% | 100.00% |
| 48 | Iron Jugulis | 1 | 1.92% | 100.00% |
| 48 | Floatzel | 1 | 1.92% | 100.00% |
| 48 | Haxorus | 1 | 1.92% | 100.00% |
| 48 | Alomomola | 1 | 1.92% | 100.00% |
| 48 | Skeledirge | 1 | 1.92% | 100.00% |
| 48 | Iron Treads | 1 | 1.92% | 100.00% |
| 48 | Cresselia | 1 | 1.92% | 100.00% |
| 48 | Regieleki | 1 | 1.92% | 100.00% |
| 48 | Gyarados | 1 | 1.92% | 100.00% |
| 48 | Magnezone | 1 | 1.92% | 100.00% |
| 48 | Slowking | 1 | 1.92% | 0.00% |
| 48 | Hydreigon | 1 | 1.92% | 0.00% |
| 48 | Overqwil | 1 | 1.92% | 0.00% |
| 48 | Slowbro-Galar | 1 | 1.92% | 0.00% |
| 48 | Rillaboom | 1 | 1.92% | 0.00% |
| 48 | Gastrodon | 1 | 1.92% | 0.00% |
| 48 | Garganacl | 1 | 1.92% | 0.00% |


Code:
+ ---- + ------------------ + ---- + ------- + ------- +
| Rank | Pokemon | Use | Usage % | Win % |
+ ---- + ------------------ + ---- + ------- + ------- +
| 1 | Kingambit | 21 | 47.73% | 52.38% |
| 2 | Great Tusk | 20 | 45.45% | 50.00% |
| 3 | Zapdos | 16 | 36.36% | 56.25% |
| 4 | Baxcalibur | 14 | 31.82% | 64.29% |
| 4 | Ting-Lu | 14 | 31.82% | 50.00% |
| 6 | Slowking-Galar | 10 | 22.73% | 80.00% |
| 6 | Cinderace | 10 | 22.73% | 50.00% |
| 8 | Toxapex | 9 | 20.45% | 55.56% |
| 8 | Iron Valiant | 9 | 20.45% | 22.22% |
| 8 | Samurott-Hisui | 9 | 20.45% | 22.22% |
| 11 | Dragapult | 8 | 18.18% | 50.00% |
| 11 | Gholdengo | 8 | 18.18% | 37.50% |
| 13 | Zamazenta-* | 6 | 13.64% | 66.67% |
| 13 | Greninja-* | 6 | 13.64% | 33.33% |
| 13 | Walking Wake | 6 | 13.64% | 16.67% |
| 16 | Enamorus | 5 | 11.36% | 100.00% |
| 16 | Pelipper | 5 | 11.36% | 20.00% |
| 16 | Basculegion | 5 | 11.36% | 20.00% |
| 19 | Moltres | 4 | 9.09% | 50.00% |
| 19 | Dragonite | 4 | 9.09% | 50.00% |
| 19 | Amoonguss | 4 | 9.09% | 50.00% |
| 19 | Corviknight | 4 | 9.09% | 50.00% |
| 19 | Heatran | 4 | 9.09% | 50.00% |
| 19 | Sneasler | 4 | 9.09% | 50.00% |
| 19 | Torkoal | 4 | 9.09% | 25.00% |
| 26 | Dondozo | 3 | 6.82% | 100.00% |
| 26 | Landorus-Therian | 3 | 6.82% | 66.67% |
| 26 | Brute Bonnet | 3 | 6.82% | 33.33% |
| 29 | Tornadus-Therian | 2 | 4.55% | 100.00% |
| 29 | Rillaboom | 2 | 4.55% | 100.00% |
| 29 | Glimmora | 2 | 4.55% | 100.00% |
| 29 | Blissey | 2 | 4.55% | 100.00% |
| 29 | Iron Hands | 2 | 4.55% | 50.00% |
| 29 | Scizor | 2 | 4.55% | 50.00% |
| 29 | Hatterene | 2 | 4.55% | 50.00% |
| 29 | Garchomp | 2 | 4.55% | 50.00% |
| 29 | Roaring Moon | 2 | 4.55% | 50.00% |
| 29 | Ditto | 2 | 4.55% | 50.00% |
| 29 | Clodsire | 2 | 4.55% | 50.00% |
| 29 | Hoopa-Unbound | 2 | 4.55% | 50.00% |
| 29 | Iron Moth | 2 | 4.55% | 50.00% |
| 29 | Volcarona | 2 | 4.55% | 0.00% |
| 29 | Espathra | 2 | 4.55% | 0.00% |
| 29 | Sandy Shocks | 2 | 4.55% | 0.00% |
| 45 | Meowscarada | 1 | 2.27% | 100.00% |
| 45 | Gastrodon | 1 | 2.27% | 100.00% |
| 45 | Hawlucha | 1 | 2.27% | 100.00% |
| 45 | Weavile | 1 | 2.27% | 100.00% |
| 45 | Abomasnow | 1 | 2.27% | 100.00% |
| 45 | Cetitan | 1 | 2.27% | 100.00% |
| 45 | Alomomola | 1 | 2.27% | 100.00% |
| 45 | Iron Treads | 1 | 2.27% | 0.00% |
| 45 | Enamorus-Therian | 1 | 2.27% | 0.00% |
| 45 | Scream Tail | 1 | 2.27% | 0.00% |
| 45 | Zoroark-Hisui | 1 | 2.27% | 0.00% |
| 45 | Arcanine-Hisui | 1 | 2.27% | 0.00% |
| 45 | Rotom-Wash | 1 | 2.27% | 0.00% |
| 45 | Regieleki | 1 | 2.27% | 0.00% |
An interesting thing of note is that Volcarona and Espathra are nowhere near the top used pokemon, Volcarona barely cracks the top 20 during week 1 before falling down tremendously during week 2.
Espathra has a 0% win rate week 2, while it does have a 50% win rate week 1, the first game where it wins it wasn't even sent out, while the second game was a ditto, where the Espathra wasn't even sent out either.
https://replay.pokemonshowdown.com/gen9ou-1921880416
https://replay.pokemonshowdown.com/gen9ou-1922228674-uwxyl0b213xwi9h2fi43dbmcyqvw4grpw
This is mostly due to the fact that Espathra just gets entirely curb stomped by Kingambit, as well as Ting-Lu.
Ultimately, I'm going to stop sharing wether i think specific pokemon are banworthy or not with or without tera other than these 3, I definitely think that these 3 would be balanced in the current metagame without tera.
The rest is entirely left to speculation, we have no idea how they'd perform in a teraless metagame because we haven't played them in one.
Ultimately I'm just curious to see if the stats change at all as the tournament goes on, I think this proves above all that these pokemon at least seem to be entirely balanced without tera, nothing seems to suggest that they'd be game breaking. Now imagine we'd be in a meta where the rest of those pokemon could still tera, it wouldn't boost the viability of Volcarona/Espathra/Eleki, it would offer even more opportunities to check them.