First of all, thanks for replying to my post, I mostly brought up the proposal for discussion.
Anyway my post was mostly just expanding on the guy talking about the metagame shifts.
To answer each of your points:
To be honest, I didn't know Kartana carries Sub, thought it always needed 3 attacks, my bad. But anyway it’s not really a complete Kartana answer, didn’t mean to imply it. It has the potential to abuse Kartana though if it the secret sword set.
For the heatran, once you copy it, you can set rocks yourself if you think he will switch, or you can kill it with earth power if you think he’ll stay. Or, you can magma storm an incoming lando to chip it like hell, etc. It’s mostly 50/50s, true, but it has its use. Most of the time anyway, you're forcing heatran to switch which lets you chip another mon.
Scarf being Latios's best set is debatable (for reference, both offense and scarf have the same viability tier on the set viability page, but not sure how updated that is) but it is true that scarf is the most used. You might be right about that, its just a possibility. Also, you don’t exactly need to draco meteor, you can just switch right after. Ditto forces switches, which is good with hazards and eventually you can get a hard predict and double switch into a counter or safely bring something like SD Kartana to potentially sweep.
After an SD boost, wooden hammer one hit kills bulu as ditto usually, especially after rocks (+2 252 Atk Tapu Bulu Wood Hammer vs. 0 HP / 4 Def Tapu Bulu in Grassy Terrain: 260-306 (92.5 - 108.8%) -- 56.3% chance to OHKO). So, not true
Impostered-Kyurem-B 2 hit kills LO Kyurem-B after rocks, which forces it to switch. Yeah Kyurem-B does like 60% back, which is admittedly pretty terrible, but you can usually get recovery with softboiled/recover/roost later in the match by predicting a tox/chansey/mantine/whatever switchin, or have a healing wisher on the team. Though you are mostly right for this point. This is mostly a last resort answer to Kyurem-B lol.
I wasn’t aware DD mage was declining, last time I checked it was used pretty often. To be fair, last time I checked veil was pretty popular, and now veil is a lot less used with kartana’s recent usage, so you’re probably right. But anyway, you still heavily cripple mage and most of the time kill it assuming you didn’t blindly let it set up without doing any damage to it. This is going to be a wall of text of math, but TL;DR pretty sure ditto still wins even after cm boosts.
Assuming Mage gets boosted +2 in special attack and +1 in special defense (lets say it calm minded and killed something), Ditto copies it and does a minimum of 40 first turn with this boost. After each cm boost, this number then decreases to 29~, and then 23~. Keep in mind, it always outspeeds unless the Mage decides to shift gear, which is pretty stupid because then it takes more hits and gets killed a turn faster. Since these are the absolute minimums, considering roll chance / chance of paralyzing, you are in favor of doing a fuckton to mage. The relative maximum damage rolls after each boost adds up to around 110, whereas the relative minimum damage rolls after each boost adds to around 93. Keep in mind, it’s pretty rare for a mage to have set up without taking any damage whatsoever. The reason why ditto doesn't completely beat it is because leftovers, yeah, but after lefties the minimum damage rolls adds up to 82ish and maximum damage rolls adds up to 97. Again, I doubt you could ever blindly let a mage set up without doing any damage to it, so ditto is actually in favor of killing it. If it doesn't kill, then I’m pretty sure you can just revenge kill the mage with any priority move since it will have like 5 health. Tell me if I missed anything in my calculations. Though, it is true that it's pretty terrible locking yourself into tbolt if the opponent still has ground types. I think you can go for 3 ice beams and go for a freeze, which is about a .27 chance to happen, but yeah that's just haxing.
Yeah the magnezone point was stupid as hell and idk how I forgot about volt switch. Honestly I was pretty high when I made that post so ignore that rofl, I was just thinking theoretical stuff. To be fair, HP Fire does around 30 which is a lot more than you would expect, but yeah nothing stops it from just taking the 30 and volt switching.
Also yeah I never implied ditto needed to copy dragon dance in order to beat zardx. Dragon claw kills anyway without the boost, and dragon claw + scarf is pretty cool lategame once you clear the fairies and steel types. Anyway it forces a switch essentially, which is useful, as you can normally just double switch the ditto.
As for the last section of my post, I figured it was kind of self-explanatory imo. You deter 90% of setup mons, you can look at copied opponent sets which lets you establish a wincon easier (for example, checking if mawile has tpunch /firefang /knock off to see if toxapex can be used as a counter), smeargle webs offense gets shit on as ditto is practically unaffected by webs, and you kill stall as ditto has infinite pp as long as you have a good enough team which can support it switching in constantly.
Ditto has a pretty damn good matchup against SD mawile by the way. If mawile ever SDs, and ditto steals the boost, it forces the mawile to switch pretty much. Fire fang and play rough at +2 both one hit kill Mawile, and not a lot of stuff in the meta can reliably tank +2 play rough mawile.
So yeah, Ditto has a LOT of shortcomings and again I don't think its very good. But its niches are more useful than other mons in its own tier. Alolan Muk, whose literal sole job is to trap tapu lele otherwise it's practical deadweight since it sucks so much as a special pivot. M-Garde doesn't even have a niche, and I can't see why M-Garchomp is better than regular chomp with an item. I’m not the biggest ditto fan, I’m just curious as to what its viability rank really should be. I think its very powerful with certain stall variants, and relying on its team to support it doesn't make it a worse mon. Otherwise, Zygarde subtect w/tspikes support sucks.
Edit: Found some saved replays that show ditto forcing switches (good w/tspikes) + being a useful wincon. Honestly not the best replays, tons of cringy misplays + not high elo, but I don't really have anything else because I never bothered to save them. These were in the mid 1600s.
http://replay.pokemonshowdown.com/gen7ou-641254274
http://replay.pokemonshowdown.com/gen7ou-640886687