I have to disagree at that point. The actors are invested but the characters are just living easier lives. El is completely different in personality to Millie Bobby Brown. She still seems this mix of both innocence at the world and naivete in social interaction but shows incredible power and strength in the heroic moments. She's not in danger from the lab. Lucas now is the guy in the relationship giving out advice. I'll take that over the first season where he was a total asshole to El or season 2 with fighting with Dustin over Max and the threat of Billy hanging over him.
My feeling is characters change and develop and complete their "arcs" sooner than others. That's what happens in life. The truth is as much as we love characters on TV shows and movies and say we want to see them happy, happy, content characters are boring. Fans complain "Why can't the writers let our faves be happy?" Well because you wouldn't give a shit about them anymore. I remember a lot of Office fans stopped caring about Pam once she finally married Jim. Even hating that her hair and clothes were better. It's only when her marriage seemed to be in trouble in the last season suddenly were back on the Pam train "Oh my god, poor Pam! What's going to happen?"