It will end after 6 seasons

I doubt it comes from the stars. Most of what I've read from actors, they recognise they are lucky to get on a long-running show because it's steady work. Generally they seem to prefer the series go as long as possible, unless there is something about the job that is untenable. Sometimes you get an actor who wants to jump from tv to movies (David Duchovny?) but from everything I've been reading recently, tv is where the good stuff is being made, and movie actors are switching more to tv. It's hard to say, though.

I suspect the bigger effect is because the longer a show goes, especially a hit show, the more money the people working on the show can ask for. I love The Expanse: yes, the characters and story, but also the opening theme, the way space is portrayed, the way they keep to the zero-gravity feel by the actors' movements, the sound of those magnetic shoes on the ship floors. But looking at it, you can tell it costs a ton of money to make, all those artificial environments and special effects. I can't imagine they could keep doing that forever, though I would love it if they did.

Maybe we'll get lucky and we'll get a spinoff with Cara Gee. That would be pretty great.