I mean, I think it's a bit crazy to suggest that folks should give up on supporting their characters in future Smash games. I don't even really know if that's what is being suggested. I don't really agree with the approach, and I would say that folks shouldn't give up on what they want to see.
It's really just going to come down to taming expectations if you ask me. When there's no evidence to suggest upgrades will happen, there is precedent that suggests otherwise, also the fact that Nintendo is the one calling the shots, and they have been for the first season of Smash Ultimate DLC.
Things can change, certainly. I would never call upgrades "impossible", but I do feel that wanting them to happen doesn't mean that they will. I'm not saying this as somebody who doesn't like the characters that could be upgraded. I love Isaac and Golden Sun is a great game, Waluigi would cap-off the Mario cast for good, Geno is a longtime fan want. I just have plenty of wants that don't include upgrades, and I would prefer that they expand into new things rather than series we've seen already.
More importantly however, prior DLC and the people choosing it don't make me confident in upgrades. I think the way to look at it, for now, is to expect more of the same in terms of DLC picks, and then embrace the excitement if an upgrade does happen. It's not like I think people are over the top believing it...I just think some folks might convince themselves it's 100% GOING to happen, despite the fact that we have no reason to believe it will, and do have reason to think it won't.
But hey, at the end of the day, it is what it is. Folks can believe what they want. It ain't that serious. Some folks will be right, others wrong, and we all still get new Smash characters. I'd say it's a win-win.