I mean, again, looking back I think it’s a tad crazy to assume a png, Assist Trophy, or other current NPC situation would prevent Nintendo from saying, “Let’s add them to Smash!”, especially if they own the character.
However, I don’t think Spring Man being upgraded (which I think is the case here, or will be) sets a precedent just yet. It would appear to me that there is a reason Nintendo decided to go with ARMS and add it to Smash. I wouldn’t say the game is “dead”, but I think it’s fair to say that the player base dropped drastically after a particular period of time post-release. Adding the game to Smash does wonders for the series...renews interest in the game, the characters, and provides an opportunity, when coupled with a demo, to allow folks a chance to try out a game that was likely passed over by many later Switch buyers.
I would love to see Isaac, or Rex upgraded to playable. However, Golden Sun for example is not in the same situation as ARMS. Xenoblade Chronicles 2 is a bit different, but it still isn’t in the same boat as ARMS.
It’ll be interesting to see what happens going forward, but I think any “upgrades” are at a case by case basis.