The mysteries were interesting but were never the heart of the show for me, nor did I feel the show "became an incoherent mess". Sorry your mileage varied.
Anyway, if you had such a problem with the writers, why did you keep watching, or at least why did you keep paying attention to them? Barring certain factors I don't care what people have to say about their art if it disagrees with my own impressions of it. If I like it I'll pay attention, and if I don't I won't. I see little point in kvetching about how my impressions differ from that of the artists.
And I didn't realize anyone compelled you to spend hours of your time analyzing the show. Heck of a thing to do for something you weren't even enjoying.
Perhaps you could try suing them for your lost time.