I agree, and think that's all the more reason we shouldn't panic or get too worked up when discussing them.
Well... the fun factor is subjective, and if a character doesn't pass it in the eyes of the dev team, they'll probably get reworked or cut before they're even implemented. It's entirely possible that's what happened to Chorus Men. So... that's a pretty nebulous qualifier for those already in the game.
And I can address those examples. Mewtwo was probably prioritized so low because he was quite unpopular in Melee, and Jigglypuff probably returned over him owing to the cache of being the aforementioned "longstanding" character. The only time a character who showed up in multiple previous titles was cut, was due to technical limitations. Which signifies sticking around helps chances.
And Doc was a clone, so he was prioritized lower, as many derivative characters typically are. The only characters cut that wasn't a clone from Melee to Brawl was Mewtwo, two of the three "low priority" characters were derivates, and most of the Forbidden 7 were also clones. Again with 4, were it not for technical limitations, the only cuts would've been derivative characters and a third-party. Some people argue that because they're less intensive to make, they'd be higher priority. But that goes against how Smash tends to view them, as more expendable than original characters.
So, we don't have all the answers, but it's not complete blind guesswork either. We have directions and we pursue them. We might be wrong, but that's how speculation works holistically, not just with cuts.
I don't have an issue with people who don't want to talk about cuts. They need not join in, no one is going to force them. No one has to talk about anything they don't want to. The only problem I have, and not to make specific accusations, is if people try to stifle the conversation even if everyone is being civil, just because they don't want to dwell on the inevitable eventuality, or if they conflate negativity with hostility.
So long as people can just accept a conversation and no be a party to it should they not wish to be, no problems on my end.