From that I surmise that Amazon invested quite a bit of money in the show, but it may not have had the effect they were hoping for. The show in itself is surely popular enough, but do people who come to watch the show on Prime Video stick around and branch out to other Prime Video shows? We have so many streaming services these days. Personally, we just pick and choose which shows we like anywhere and watch them, irrespective of what platform they are on. I suppose this is a luxury we can afford due to being subscribed to pretty much all the main services. I read the other day that the average American is signed up for 8 streaming services. That is quite a lot of choice. Maybe the costs of producing the show are outweighing the rewards in subscriber count. There is also the fact that this has now become an older show and generally those tend to lose some viewers over time whilst not picking up many new viewers. SyFy used to be notorious for cancelling shows after five seasons, even good ones. Maybe they think they've milked this show for what they can. To be hones, I myself found seasons 4 and 5 to be much less interesting than the first 3 seasons.
I pay for Netflix, apple, amz- but of course that has the faster delivery service as well, and through amazon at a few dollar discount i get epix, britbox, acorn and paramount (i discovered they do have some other shows that are pretty good- like CLARICE and EVIL)
also pay for VIKI where my wife gets content from several korean TV channels
it is still FAR cheaper but far more rewarding than paying for satellite or cable
and than all the free stuff from/through places like youtube, roku channel and pluto -- never regret being a "cord cutter/ dish ditcher"
=============
but to your point of favorite shows being all over the place
i can see how someone may make- like say ROKU or anyone else who makes streaming devices or just some app developer- makes some kind of aggregator that serves as a reminder and scheduler that puts all of your watched and followed shows in one view-- AND THEN allows you to access those shows straight from that app
-- maybe someone HAS already done this?