Not sure how 8 players in one game affects another game on another system. Sure, they're similar games, sure, Smash is better, but what does the 8 player feature have to do with it? It's a special mode that isn't available in all circumstances. It's awesome, but a single feature won't singlehandedly kill another franchise. No reason Sony couldn't copy it.
But yeah, Smash isn't why PSAS died. PSAS died because the game sucked. Sony made a quick buck off the IP rights by turning it into a mobile game, then cut off the developer. They're done with the franchise. It's a shame, because I really want a Playstation mascot fighting game, and all I have is PSAS. Sony needs to get Crash and Spyro back, make a new, better PSAS, and use it as an excuse to make more games with their classic IPs. New Crash, Spyro, Ape Escape, Jak, Sly, and Ratchet. Throw in a new PaRappa and MediEvil. All on the PS4 alongside a good PSAS. Watch the PS4 outsell the PS2 by 2:1.