Ask yourself questions like this.

How quickly do new characters overshadow old characters?

Do older characters get buffs? If so, are the buffs enough?

Are older characters still able to clear all content, without an insane build?

And yes, I'm copying this to the other subreddits since there isn't a general Hoyoverse subreddit, as far as I'm aware.
In terms of powercreep with old units, how well would you say each hoyo-game handles it?