I think a lot of arguments around allowing flutter depend on the meta adapting to it, but I want to ask, how? First off, physical attackers are not, and never will be, a fully reliable answer to Flutter. Not only does flutter have countless partners that can easily resolve this issue, redirection from ogerpons and amoon, incin, partners like chi yu and lando that can easily ko most of said threats, torn for speed control, niche options like tsareena and ndd (the latter also providing redirection) to stop prio spam, but also, flutter's stats are so insane it can easily invest massively in bulk and it can take a shocking amount of hits. Further you have incin and other options like screens to shore up its bulk furhter. I've also experimented with cm grassy seed flutter to pretty decent success. Drawing from my experience from vgc, there is no way to reliably check flutter, and it can ALWAYS adapt to its checks. I've seen it time and time again, flutter making a new set to beat its counters. Even if you manage to tech around flutter's physical damage reduction like with defiant kingambit, then you just thud into regular offensive flutter + chi yu cores. Additionally, flutter outspeeds the entire meta, causing an insane mess of speed creeping and icy wind wars between opposing flutters, which I find really unhealthy. (Particularly when ppl catch onto twave flutter). I can't think of a single mon or core of mons that can reliably check both supportive/defensive flutter + a common partner like incin and offensive flutter + chi yu/lando (heatran dies tospecs shadow ball + dark pulse and loses damage wars if you play around it). And anything that might is so specific that the meta is centralised around it to an almost insane extent, but unlike lando t or other similarly central pokemons in past metas, flutter can win games by itself, and exudes an immense offensive pressure whilst being the fastest thing in the tier.
Further, the few mons that are good in the tier outside of checking flutter, like rilla and tera woger, are almost always used to damage trade with other massively dangerous foes, and thus are very vulnerable and easy to chip down for flutter, or for flutter to chip for its partner. Flutter can activate so many offensive cores whilst seemlessly being a game ending threat in itself, whilst it can slot with ease into common balance builds and single handedly ruins so so many otherwise strong and viable options within the tier. Flutter is an obviously unbelievably unbalanced mon, and it has proven time and time again in formats of a much higher power level then this that it has the tools to adapt to whatever you attempt to check it with, and controls the game in a way no other mon currently legal can.