The Leftovers is indeed aware of the show formula I'm critiquing, it's even in the theme song: "guess I'll just, let the mystery be." This is Lindelof indoctrinating the viewer to accept his style of writing, which doesn't burden plots with conclusions. He's still frustrated we asked 'why' of the LOST plot, to which there was no good answer. The Leftovers similarly offers no good answer to 'why'. Making that the point doesn't stop me from asking.
If the show was doing well HBO would have signed a fourth season and they'd have left enough room in the third for yet more surreal diversions headed nowhere. Online review aggregators are meaningless.
You asserted the show had "an engaging pilot episode with only a back of the napkin roadmap for the rest of the show", which clearly isn't possibly the case for a show whose entire first season was adapted from a novel.
The third season was written as a final season. Regardless of whether the mystery is resolved, the show's plot and character arcs firmly concludes in the finale. Professional tv critics nearly universally view it as one of the best seasons of any television show ever made.
Yes, there are unanswered questions. This is obviously intentional (although the theme song you reference was only used in season 2 and the finale in season 3). Similarly in real life you'll never find out which religion, if any, is correct. Or why the pandemic happened. You can still ask, and writers can still explore these topics, even though no one ever knows.
If you didn't like the show, or ambiguous art/media in general, that's fine, it's not for everyone. That doesn't mean it's all part of the same capitalist formula to produce bad art.
As an interesting counter-point, consider Twin Peaks, a show where the network demanded that the creators promptly resolve the core mystery at the end of the first season -- much to the show's detriment.
If the show was doing well HBO would have signed a fourth season and they'd have left enough room in the third for yet more surreal diversions headed nowhere. Online review aggregators are meaningless.