So, cannon and serialization aren't the same thing. Cannon is the general mythos of the show, while serialization is the method of storytelling. Specifically, it's having a story unfold over many episodes, a season, or even several seasons. Dr. Who treats its cannon very lightly, but the show is fairly serialized, with small hints being dropped throughout the season on larger plot lines, even in stand-alone episodes (the Pandorica, the Silence, Bad Wolf, etc.). It also has large changes that last from season to season, regeneration being the most obvious.
Compare that to TOS, where everything is a stand-alone story and all the characters return to their status quo positions at the start of the next episode. Pretty much nothing carries over from week to week (except Harry Mudd, I guess). You could watch every single episode of TOS out of order and it would make perfect sense (aside from the two-parters, obviously). If you watched every episode of Doctor Who out of order, you'd wonder why the companions keep swapping, why David Tennant keeps getting replaced with Peter Capaldi, and God help you if you're trying to follow any of Moffat's later episodes.
Lower Decks is pretty serialized, with things like the Texas class ships and the Pakleds developing over the course of or in between seasons (the Locarno storyline is probably most involved of these). But, aside from story, there is a lot of character development that goes on over the series. Mariner has a completely different relationship with her mother, Ransom, and the Federation now than in season 1. Boimler is more self-assured and less obsessed with rules and rank. D'Vana is more open about her Orion upbringing and even changed career tracks. There's a lot of growth and change compared to the characters in TOS.
So, I'm saying that if they keep going for too long, they'll either have to promote these characters out of the lower decks or it will be weird that they're still stuck at menial ranks. You seem to be saying they should just place the characters in a state of arrested development and only have them, "go be themselves," in wacky adventure-of-the-week stories. I think that would be a very weird direction for the show to take after giving them 4 years of character growth, and I'm willing to bet most fans would feel the same. If you don't, fine, then we just have to agree to disagree.