Sad thing is: if I go back about 15 years, Marriott was at least super-consistent and had well-trained staff and generally good service across their portfolio. The Marriott flag meant something in a world where the other mainline business hotels were much more variable. (Although I was SPG Plat as well, I somewhat avoided Sheratons for this reason.)

Marriott Rewards was never a big suite upgrade program like SPG was, mostly because the buildings often didn't have many suites to begin with, but you could absolutely count on a seamless check-in process, no hold times on the Gold/Platinum phone support, restaurants and bars that were open and delivering a decent product, adequate executive lounges at business hotels, generally nice rooms, and quick resolution if anything went wrong.

If SPG was Nordstrom, Marriott was at least a clean, well-run, properly-staffed Costco.

They took a thing that worked pretty well and broke it. Plain and simple.