end of the day if the show was moving merch for amazon they would have kept it going, its not a big shock that the show kept losing viewers season to season so it made sense to end. contracts start getting ridiculously expensive past season 5 of any show. The show was just boring the last 2 seasons so there's nothing to suggest it would have suddenly improved in quality if they finished the last 3 books, and its nearly impossible to attract new viewers nowadays to shows that go on forever.
Thats why the new thing is just constantly spinning shit off, why Walking Dead has like 5 spinoffs all resetting with season 1, Narcos kept spinning stuff off, Showtime spins off their crime shows, etc.
I guess you could argue they could film books 7-9 as a new series, The Expanse II or whatever, with even a new cast but ... why bother? There are a million other scifi IPs you could start fresh with and be even more popular like Commonwealth, Culture, on and on.