While the idea of Smash being a "celebration of video games as a whole" is certainly idealistic, it's simply not practical. Even if by some miracle, Xbox icons like Master Chief and Steve (I guess, I'm not familiar with Xbox) got in due to Microsoft's relationship with Nintendo (highly unlikely, Xbox still has brand integrity to protect), PlayStation icons like Kratos, Sly, and other Sony-owned characters are firmly impractical (I refuse to say impossible). It is my personal opinion that Smash should've mostly remained a Nintendo-first series, but it's clear Sakurai doesn't share the same idea, viewing it more as trying to build the biggest crossover ever (personally, I would've never allowed characters like Bayonetta or Joker, although the other third-parties make sense). Not to mention, there are only so many characters you can get before you'd need to reach back into Nintendo's catalogue once again. And once that occurs, Isaac is at the forefront. I can't see Nintendo looking at the fan backlash to the AT as well as Golden Sunday, then going to Sakurai and saying, "well, we'd love to put in a fan-demanded first-party character that would cost us literally no money to acquire, but we'd rather get [insert Sony character here] for [insert absurd amount of money here]." Plus, Sakurai must be aware of the demand, and he does have basically the final say when it comes to DLC. Isaac's name had to have come up at some point, and I think it was at the perfect time for it to all come together.