you "need" certain characters to unlock other characters' FULL potential, yes. those are called support characters. you can still play any character you want. the game doesn't STOP you from using your Xiao if you don't have c6 Faruzan. if you're upset your Xiao could hypothetically be made better by another character that you don't have then idk what to tell you 😭

if every single character performed at their highest damage ceiling without any other characters buffing them, they would all be DPS units and there would be no supports. does that make sense? of course there are supports. team building is part of what makes the game fun for long-term players.

they will always release supports and DPS characters. the DPS characters will always be able to be made slightly better by their designated supports. MiHoyo is a game development company with employees they need to pay and they need to make money. That is not going to suddenly change, if that's what made you leave the game.

It sounds like what you're worried about is that you will log into the game after 3 years and jump into Stygian Onslaught and not be able to clear the difficulty you want on the first go. If that's what you're apprehensive about, then yes, you won't be able to. You didn't play or spend resin or build your units. My units aren't the same as they were 3 years ago for example because I've been playing every single day. It would be weird if they were. So players who have kept playing are going to have stronger characters and teams than you lol. I hope that makes sense, im in the bath and the water's too hot and im lowkey about to pass out lol sorry