OTOH I could see Sony (even if reluctantly) agree to a side game or a port of an older game if it means a Smash appareance. Yes, even when it comes to the big guns. There are a couple things worth noting:
* PlayStation AllStars is dead and buried. Yea, Battle Royale didn't exactly get off the ground running - but what sunk it IMHO was the Coke Zero sponsored mobile game Island. Sony doesn't have a competitor to Smash alive, so it's either Smash or nothing, Likewise, Sony's direct competitor nowadays is Microsoft - not Nintendo. Sony doesn't have a handheld, Nintendo doesn't have a 4K power machine.
* Sony recently (and quietly)
agreed to crossplay. We'll see if this is just a PS4 thing or it'll continue into the PS5, but that's gnawed at them for a bit too long. Microsoft - and Nintendo - got a lot of mileage out of that, and I doubt Sony wants to lose more ground on that front.
* Sakurai saying "Banjo Kazooie is currently available on the Xbox One" is not the first time. What with Metal Gear Solid 4 being a PS3 exclusive and Brawl still pushing MGS4 quite a bit. If there's one developer who could concievably approach Sony it's Sakurai.
Even Sony's dormant or more minor franchises are notable - Sly I'd argue is around as popular as B-K overall (outside of Smash) if not slightly bigger. Gravity Rush's in a decent position, Medievil got a remake recently and even though LBP's dormant it's still a recognizable series,
Whether Master Chief getting in or not will be key, since if he does Sony will be on the outside looking in while Nintendo and Microsoft goes all in on Smash. Steve would also be a similar warning shot for Sony, and it becomes a question of "Get nothing and watch your two competitors gain a ton" or "Watch your two competitor gain a ton, but also gain quite a bit by getting your characters in Ultimate (and promote your own game) as well".
Not saying Sony getting in is likely - I know it's very unlikely. Especially since Microsoft has one of Nintendo's former second parties, something Sony doesn't have. I'm saying that the groundwork's partly laid already for a potential turnaround.
*Looks at
Nathan's PSASBR moveset*
*Sees concrete barriers, oil drums, tranqs, ziplines and stone pillars. In case Sakurai vetoes real life guns.*
Turns out there's actually something there for a potentially interesting moveset.