Remember, we're talking about Sm4sh, as in the wii U and 3DS ports, not SSF2.
I think Sm4sh is dead because you don't see as much activity around it on Youtube or other places anymore. Tournaments aren't as common, and the footage doesn't generate any hype or reactions from the crowd. We don't hear a lot of people talk about champions like TSM Zero or Ally anymore, and lately even they have been getting wrecked in tournaments by bayo mains.
There's also no more excitement behind exploration and discovery of new techs or strats. We've pretty much figured out that Bayo is the undisputed champion of the tier list with Cloud and Diddy Kong tailing behind as counterpick characters. Even tournaments that are famous for featuring low tier champions are being dominated by the Bayo meta. The most recent patch tried to nerf Bayonetta, but she's still at the top of the tier list.
Some of our favorite Sm4sh youtubers, namely Alpharad, Lythero, and WeeGee plays, have talked less about Sm4sh, and lately have gone to discuss other games. Introspektive is still hanging on, but he's dull and won't be much help to the fanbase. Pretty much the only hope for the Sm4sh fanbase is Izaw, but I doubt he really has the content the demographic of Sm4sh looks for.
We'll pretty much have to hope that Sm4sh gets a following similar to Melee, where there are fans devoted to the game even 15 years later, but seeing how a lot of people don't appreciate how Sm4sh runs slowly in comparison to Melee, that's not likely to happen.
Just now I played online and hosted a room called "Sm4sh is dead" and I got significantly more requests to join than I normally get when I name the room something like "1v1 NA West". This tells me that there is already a consensus that Sm4sh is losing steam and may very well be lost from the hearts of the SSB community.
So I ask you, with tears rolling down my cheeks, do you think Sm4sh is dead?
_________________"Why Hello there, Dream Raven!"
-HaramBABY (Slay.One)