It isn't about points against it or what have you. When Sakurai selects stages he looks for two things; stages that bring something different than any other stage, and reflect the overall series it represents. Sin and Punishment: Star Successor was very well received by reviewers and most everyone who played it, it's only problem was being a niche series on a platform that does not accommodate those type of games. That, however, is beside the point.
Don't get me wrong, I'm not trying to downplay Sin & Punishment as a series, the fact that it would make an interesting and appealing stage, or its character chances, I just think it's not on the same level as something like Animal Crossing or the Nintendo DS, which even without playable representation, still has enough popularity and importance to warrant a stage. Sin & Punishment really doesn't have the audience capacity that the two others do, so if a stage without playable representation was to be included, it would most likely be due to Sakurai's bias (unless for some reason precedent changes and Sakurai includes stages from multiple unrepresented series). While Sakurai's bias is a good way to increase the chances of something (it might have been why we got an Electroplankton stage, which I'm still not sure why we got, and honestly weakens my argument here
![Awesome :awesome: :awesome:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
) I don't believe it is enough of a factor to cause something like a Sin & Punishment stage to become inevitable.
There was a time before Brawl back during the days of Nsider where no one believed that a stage would ever not be associated with a character, so when Smashville was revealed on the DOJO it led to a great many arguments and debates. We all found out it could happen. Just because something was true for Brawl does not mean it won't happen in the next game. Yes, precedent is always the better leg to stand on, but it isn't the only one. Just because something hasn't happened does not mean it will not.
You're right. Technically it could happen, there might be a chance we will receive a stage from a series that doesn't have a corresponding viable character to go along with it. But I think it's just that, a chance, not a guarantee, as you seem to think. While precedent can change, I believe the fact that it hasn't so far in the Smash series proves that even though there is the possibility of it changing, without any evidence yet in that favor, it is still a large assumption to make that it, without doubt, will change.
As for the rest of your points, I pretty much agree with them, a Star Successor stage would be a really cool stage, and I would love to play on one. I personally liked the game and would be happy seeing it in Smash, I just think it isn't as likely to be a stage without a corresponding playable character as you do. I'm not against the idea of S&P representation though, if anything, I'm advocating for more.
And I wouldn't worry about Sakurai getting it to fit in Smash style, after what he managed to do with a series like Metal Gear, like you said, I'm confident he'd be able to get pretty much anything to fit if he really tried.
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Also, I'm not into all the Tekken characters in PSABR, though they do fit among most of those characters more than among Nintendo ones. Ryu Hayabusa I could go either way on, but it's both funny and sad to see how much they're relying on 3rd parties.
edit: It's also hilarious how much the game's roster is leaking.