I don't know why they would include it if they don't have plans on continuing the show in some form in the future. It's not like there was any shortage of content in the mainline story to cover.
This is not the mark of a show that they plan to renew. :-(
Amazon doesn't own the TV/movie rights of The Expanse (beyond the seasons they funded), they're owned by Alcon Entertainment. So basically, whatever Amazon does or doesn't plan to do is kind of irrelevant and there isn't really anything stopping from the show appearing on, say, Netflix, or the BBC in a few years time. This is also why it transferred from SyFy to Amazon relatively easily.
I suspect they're going for a film trilogy with a much bigger budget per minute filmed than the TV series. For one thing, some of the action gets a lot bigger in scale. Two, the time jump means all the main characters are now in their 70s and 80s. Avasarala is over 100. Either they go the Twin Peaks route and actually wait 30 years and hope everyone is still alive and working, or they go the Disney route and spend ungodly money on digitally aging everyone. Or they could just recast, but I think doing that as season 7 of the same show would have been pretty jarring.