Roblox has lasted so long because it's not a specific game which is only relevant to a particular audience, it's a platform for whoever the users are to create and share games/content with others.

I never really liked it because of the virtual economy aspect, but the only link between Discord having quests on Roblox or adults playing Roblox and kids is that's the majority of the userbase and therefor content, not that it's only for kids.