I gotta disagree with you there, I wouldnt call a few years of popularity a legacy. While I dont have the data in front of me, I’d be willing to bet Overwatch’s playerbase has dropped significantly (not taking the Hong Kong controversy into account). I myself stopped played about year in, as well as most my friends. I dont know anyone who still plays Overwatch irl regularly anymore. I’ve moved back to TF2 and I think that playerbase has actually grown a bit recently. This is just circumstantial though.
I’m not sure if Fortnite will have the lasting appeal, but I’m sure the younger crowd will be hooked for awhile. Whether they come back to it when theyre older like we have with Minecraft remains to be seen.
Safe to say that Blizzard did more to harm Overwatch over its run than Overwatch ever did, but we have to define "legacy" when we think in these terms. Sure, it's been two years, but Fortnite's legacy is already to have one of the highest payouts (if not the highest) in competitive gaming. It's one of the most played games in the world, likely owing to the fact that it can run on a toaster and has come to every major console as well as mobile, and despite not inventing the genre, has defined it.
I want to cut my hands off for typing praise for this game.
This kind of legacy, though, doesn't mean much as a video game. Your last statement about whether or not these kids are going to come back to it like Minecraft, though, the answer is probably no. Not because they won't want to, but because there's no way Fortnite or Overwatch can continue longer than the developer intends for them to. The games are a multiplayer service, and not really "games" by a standard definition. Until the last hard drive with Minecraft on it succumbs to bit rot, Minecraft will live. As it stands, you're breaking the law to have a private Overwatch match (since it requires a cracked copy of the game and everything), and I don't think hosting your own Fortnite server is possible, either.
That's one thing about Smash characters - they're all from games that are complete single player experiences, many of which never had a multiplayer to begin with, and the ones that do aren't pure multiplayer. Splatoon is the closest to multiplayer-focused game represented in Smash, and even then, the only paid DLC it's ever had is a single player campaign (which was received incredibly, might I add). I think it might be safe to say there will never be a character in Smash who will die when their last server goes offline.