I agree with your deconstruction: these shows are optimized for selling an engaging pilot episode with only a back of the napkin roadmap for the rest of the show. The finale comes only when there's no more money to be made.
I'm very excited by writers like Baran bo Odar and Jantje Friese who are able to write interwoven plots comparable to LOST, but with cohesive endings. Check out Dark for an example.
The novel doesn't resolve the core mystery. I won't spoil whether the show does, but it's kind of beside the point anyway. One of the core themes is how people grapple with sudden loss, something beyond their control, something unknowable. In any case the show resolves itself with a satisfying amount of closure in three seasons. Much fewer open threads and plot holes than LOST. The unresolved aspects of The Leftovers are genuinely thought-provoking and deep.
I wholly agree with Wikipedia's summary: "The first season received mostly positive reviews. However, the second and third seasons were highly acclaimed, with many critics referring to The Leftovers as one of the greatest television series of all time, with particular praise for its writing, directing, acting (particularly Theroux’s and Coon's) and thematic depth."
You say "The finale comes only when there's no more money to be made" ... that doesn't jive with a final season having a 99% on Rotten Tomatoes and 98% on Metacritic!
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.
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.
The continuing story of Lost (and similar series) was a desperate improvisation being written by hired guns of whom there's no reason to think that they would have any more affection for the show than a random person off the street.
For an on-the-nose example, check out the recent, similarly-named FROM: https://en.wikipedia.org/wiki/From_(TV_series) For a fun (<- minority view) absolute commercial and storytelling disaster version, check out Aftermath(RIP): https://en.wikipedia.org/wiki/Aftermath_(2016_TV_series)