I voted no ban on Diancie-Mega because of 4 reasons:
1. Out of Primal Groudon, Kartana and Mega Diancie, it is by far the least impactful in a standalone meta where the other two are banned.
All you really need to check it is a RegenVest steel and Imposter/PixiBurst sponges paired with a few pokemon who can tank a coverage move and safely kill it. It may sound like a bit much but practically every competent team does this anyway, its not hard to do with good teambuilding. I'm also open to retesting it in the future but I doubt there'd be enough community support for that.
2. I've never seen a single game where Mega Diancie rips through teams and is actually as scary as people claim it is, most of the time competent players using fat just outoffense, preventing it from getting any actual switch-ins and don't let it do much damage. This isn't a new case either, Mega Garchomp was similarly unwallable in Pre-VC meta with good play, yet no one ever had serious complaints about it because there were many options available to play around it, just like there are many options to play around Mega Diancie now. Feel free to send me a replay where a council member gets thrashed by Mega Diancie while using a competent team if you think I'm wrong about this.
3. Mega Diancie is singlehandedly holding up the "outoffensing" meta because of its revenge killing capabilities. This is reflected by the widespread popularity of Psychic Surge HO this meta, a strategy that primarily exists
because the style struggled so much with a common
-atespeeder being in the meta. Psyspam HO was a very annoying presence in the tier, but at least it was limited by its overreliance on Mega Alakazam and Seed/Torch Unburdens generally being weak enough that Strength Sap FCs and Dragon Tail RegenVests respectively could reliably 1v1. Without Mega Diancie these teams will be free to use much stronger pokemon like Speed Boost Palkia, Speed Boost/Magic Bounce Mega Garchomp and Flare Boost Flutter Mane, making offense significantly better and forcing players to prep more to beat it. The only way to beat these kinds of fast setup sweepers will become walling them. This
limits teams that can't afford to fit their walls like Topsy FCs or Ice Scales because of how passive they are, forcing them to just accept the bad MU in the builder instead of changing one of their breaker slots to fix it. I know most of you haven't seen these pokemon in months so here are a few replays as a refresher
https://replay.pokemonshowdown.com/smogtours-gen9balancedhackmons-886919?p2
https://replay.pokemonshowdown.com/smogtours-gen9balancedhackmons-889935
https://replay.pokemonshowdown.com/smogtours-gen9balancedhackmons-893359
https://replay.pokemonshowdown.com/smogtours-gen9balancedhackmons-902744?p2
You'll notice that none of these replays are from our current metagame (also that my HOs weren't always just psyspam lol), this is because Mega Diancie's mere existence in the tier discourages the use of these types of pokemon, as the arduous process of creating setup opportunities can be made useless with the threat of PixiSpeed easily, leaving the setup user vulnerable to losing first. I say this because these types of pokemon will become more popular should Mega Diancie be unbanned,
making teambuilding more restrictive than not. Some of you might be asking "If these setup sweepers are so broken, why not just ban them?". The problem with this idea is that doing that would require banning Nasty Plot, Speed Boost and every Dragon Dance clone. I don't think you need a council member to tell you that's never getting enough traction to get passed so we'll just have to deal with it.
4. Banning Mega Diancie would additionally encourage the matchup fishing breaker microgames we had in pre-VC meta, as if wallbreakers that can't reliably some force progress into every defensive core aren't in the tier, players will be forced to fish with the breakers that are available to do it instead. I know this is already a part of the metagame to an extent, but at least now both sides have access to such powerful wallbreakers that its possible to win faster and outplay bad matchups. If this happens,
nearly every game will be decided at lead based on which teams breaker has the better matchup into the opposing teams defensive core, or both teams can't make any progress which lead into 200 turn games that only end if 1 player gets haxxed so hard they can't play anymore, like an Ice Beam freeze in a TH Arceus war or a lucky crit on your opponents dedicated FC or RegenVest. I don't believe these problems can go away without the presence of powerful, reliable wallbreakers, that still require thought to use in the tier like Mega Diancie. Attempting to ban any pokemon that's good at breaking balance will only lead to non-games decided by who prepped better.
TL;DR
Mega Diancie is the least broken between it, Primal Groudon and Kartana
Mega Diancie can be played around and prepped against much easier than people claim
Mega Diancie plays a crucial role in limiting the usage of fast setup sweepers that are typically very annoying for most teams to deal with
Mega Diancie actively prevents the meta from regressing into an extremely MU fishy state