Maybe I'm being too optimistic, but don't let this deconfirmation discourage you guys. You made it far, and gave Isaac and GS a lot of attention. Probably more than it's had in years, possibly ever. Let's not forget that Isaac headlined the Assist trophies. That's a jester of recognition by Sakurai. He recognizes the demand for the character and is displaying it in the best way without him actually being a fighter.
What I am getting at is, Smash Ultimate is a huge game, which fulfills many dreams. So much so, that I was starting to worry how the next Smash will top it. And let's be real, there will be a next Smash. Nintendo and Sakurai had to weight their options. If almost every popular newcomer was chosen, what would Smash 6 have to offer as an incentive for hype and speculation? Who gets cut? That would be a bummer. Nintendo hasn't created that many new properties, and they are running out of big new series to push. Eventually, they'll have to start mining the well of forgotten IPs, like Golden Sun. It's up to the fans to let them know it's an investement worth making.
In conclusion, don't start wavering. As a K. Rool/Ridley supporter, I've gone through this. Never give up, make your voices heard. Don't just let Sakurai know you want Isaac playable, let Nintendo know you want a new Golden Sun. It's never too late. I'll be helping by playing GS for the first time. I'll even tell friends about it, and spread the word. Don't lose faith.