100% going to happen in the worst way and they know it.
So long as they have one above and beyond show every time they will be fine.
Right now they have a limited selection of high quality shows, just enough to keep people hooked for the next great one. If they switch to just pumping out lower-quality, higher-quantity stuff, I suspect people will just cancel -- everyone already has Netflix for that.
HBO seems to think it's the next "Netflix", but if they follow the strategy they've described so far, I suspect they'll be the next "CBS All Access".
AT&T seems to think HBO can be HBO and also be Netflix; they're wrong, but it's not inevitable they’ll break the being HBO part irrecoverably before they realize that.
Which ratings are you looking at?
EDIT: I also checked Metacritic. True Blood is rated 68 on average across seven seasons, meanwhile Westworld is rated 75 on average across two seasons. If you take the Metacritic rating peak of Westworld (Season 1) and the Metacritic rating peak of True Blood (Season 3), True Blood is only four points ahead.
Given the difference between the Metacritic score and the Rotten Tomatoes/IMDB scores, I'd be inclined to chalk it up to different audiences. But even on the preferential platform Westworld basically at parity with True Blood, not "far south" of it.
It sure seems like HBO executives think they're a cut above, though: https://static1.businessinsider.com/att-ceo-compares-hbo-to-...
I'm just not sure they can justify their "premium" status with their price point. GoT's final season is nearly here, and after that, I'd be curious if cancellations start to happen.