according to imdb the new writing staff also worked on s03e01, so if you're trying to argue their influence is somehow poisoning the remainder of the season, you'll have to try harder than that. at least make sure your sources are consistent with your argument.

as for the rest... yeesh. i don't know what show you've been watching where the characters just do shit and it has absolutely no impact on their long-term development, but it's not rick and morty. does this look like the face of somebody who is absolutely comfortable with the fact he just left his parents behind in a doomed timeline? don't you think the fact that morty's referred back to this experience more than once might go to show it still holds some kind of significance for him? the point is morty does know these people now, their choices do have an impact on his life, because he's still trying to cling to some sense of normalcy in spite of having rick for a grandfather.

and yes, rick enforces a kind of confident apathy upon himself in order to cope with these things he's done. no, that doesn't mean he carries absolutely no emotional baggage from his past. you may have noticed he is a depressed alcoholic who has been shown attempting suicide on screen.

given all of that, i don't think it breaks the tone at all to flesh out the supporting cast. like... did you think this was a new thing? did you miss rickles in time? ricksy business? big trouble in little sanchez? why would this show spend so much time on jerry, beth, and summer if they were apparently completely inconsequential and disposable? jerry alone dominates most of the b-plots. the only thing that's really changed is that beth is getting a little more focus, which is definitely a welcome development imo.