There are many things outside of tera types that one team cannot account for that will surprise you and result in a loss.
All of those surprises at least come with an opportunity cost. Weird scarfer? That's ok, they're locked into a move with no extra power. Weird resist berry? They essentially have no item until its used, which can really limit overall power/speed.
But weird tera? You can lose on the spot and the opponent loses nothing in matchups where that surprise tera isn't necessary. Tera is thus unhealthy in this regard, in that lures have too low of an opportunity cost.
I do not understand how saying that I have been swept by a suprise Tera is somehow undermining my POV that Defensive tera give you an option against team archetypes that would normally require 1/2 dedicated defensive switch ins.
Both of these things can be true. Your POV is valid, it is just less significant than the breadth of tera threats you need to account for.
This game is about creating high probability win conditions, of course you cant build around every single Gambit Tera option, but i think that could be addressed by looking at Gambit itself before Tera?
I highly doubt kingambit suspect would result in a ban, because it covers so many necessary bases defensively. We lost a single ival/enamorus answer and we really feel it already, even if prepping for volcarona was a higher burden. Kingambit would not be borderline broken without tera.
In my experience it feels way worse to lose at team preview when you match up against a team archetype that you do not have viable means to answer. My team I posted earlier is an example of a team without a hard water check that can still carve out defensive positions against Water spam with the help of an early defensive tera.
I can't really deny your experience or feeling, all I can say is this: A MU loss is a MU loss, regardless of whether you lost at team preview or realize you had nothing for x mon with y tera later in the battle. We should try to minimize MU losses as much as possible.
I think it is a little short-sited to list Gambit and Valiant as two example of oppressive Tera abusers, I think everyone would agree these pokemon would remain incredibly threatening and viable with a Tera Ban.
Gambit has a combination of insane bulk, strong typing, a busted ability, and access to one of the most over-tuned priority moves in the game.
Valiant can run 5+ viable sets plus booster Speed. Scouting which you are playing against still can cost one or more pokemon even when your opponent has blown tera.
This is speculation, but I think they would both be healthy in the tier and not borderline busted if tera was banned. Gambit actually loses to fighting types? Ival cannot suddenly get an extra turn to set up and get stab on its coverage? Much easier to handle.
I am strongly of the opinion that we SHOULD try and preserve Tera in Gen 9 as a means to keep this period in competitive mons distinct and fresh compared to what it felt like in Gen 8.
Gen 9 will be distinct and fresh regardless of whether tera remains or not. Less knock off, less toxic, less scald (for now

), less recovery pp, no defog, stronger hazard setters, and booster energy+paradox pokemon are HUGE differences that will not change. People who fearmonger over SV becoming SS are extremely paranoid. The very fact that
Tornadus-T is MID tells us everything: we wont go back bros dw.