Yeah, there's no rule he ever set in stone at any point till, well, Smash 4, when he made the clear rule of "video game characters only". There's nothing to compare it to. There's nothing to really suggest the rule was a thing before Smash 4 either in general, but it is one now. That's really the first time where multiple 3rd parties existed, so the only time when he really had the option of where he could consider a non-game 3rd party. So it makes sense that's when he declared it. His entire reasoning has so far been "it's impossible" not "it's difficult" too, and all we know so far is 3rd parties mostly had issues with licensing more than anything. It's hard to come to any conclusion beyond that. He doesn't show any hatred towards characters or archtypes, etc. either. Which is good, cause he isn't a petty man like that.
That said, it's Ultimate when he doubled down on this as well, and gave the game a strict theme of "game about gaming characters", which as I noted beforehand, he and Nintendo came to that agreement. This means nothing for future Smash games in itself, as things can change, but as of Ultimate, Sakurai and Nintendo are on that page. It doesn't necessarily mean Nintendo is still going with that route either, as only Sakurai repeated his opinion on why they shouldn't be in(due to his specific theme for Smash Ultimate and licensing, really), but that's it.
The only chance they have right now is if Nintendo changes their mind, and that's not really that implausible. They're in it for business first, and if they see a good business venture, it's pretty easy to choose an option that benefits them. Promotional characters are one of the easiest cases of these. But if they choose a non-game 3rd party, they're going to be a big character on its own anyway. Not just "anyone". I still don't think it's happening, but that's pretty much on Nintendo now, not Sakurai. Let's say Nintendo goes for a non-game 3rd party. This barely affects Sakurai as he wouldn't go for new ones in a new game he's directing(for base roster) unless he feels that a specific one is a worthwhile addition or fits a particular character archtype/theme he wants to add. Otherwise, just that character back and that's it is the best to think on.
I also never really felt the floodgate arguments stood up. The worst that'll happen is Sakurai gets bombarded with somewhat slightly more requests, but the difference is pretty small. Smashboards gets more support topics. And... yeah, that's kind of it. At worst the topics grow enough to split up the boards slightly to have two forums, one for game support topics and one for non-game support topics. Basically, if somebody wants to go into a topic to stir up trouble about it, they're the problem, not the topic. Nor is Sakurai/Nintendo for adding a non-game 3rd party. The issue is people refusing to handling it in a respectable fashion. Disliking it is not the same as stirring up trouble cause of it. Oh, right, and a few topics clearly stirring up stuff that will get locked due to taking things too far. So... basically what already happens all the time. Yeah, I don't see an issue here cause just the fact Smash exists constantly causes people to create issues on their own. What'd you expect. The fanbase gets incredibly toxic anyway. Sakurai cannot literally make any decision at all without some kind of issues caused. So... I find the whole "problems" pretty exaggerated at this point. I already dealt with more than enough of these as my time as a Mod and I will say it was not actually hard to handle because people bothered to report posts extremely often as is. Rarely do people abuse reports too during it. I obviously can't say any names due to keeping lots of mod stuff secret either way. To put it clearly, those random "non game character" topics were easier to handle than some of stuff that is still going on. I can't, again, name names, but it gets crazy still. Nothing really changed anyway when it came to problematic topics.