but I'd put the rot starting between 2008 and 2012.
Disagree, not in with your point in general but with this specifically: That is the exact time period in which triple A actually became a thing in the first place.
So you are actually saying that the rot started with triple a which leads to the only correct conclusion: Triple A was a mistake. But maybe it wasn't.
Sure, cinematic shooters like COD and moh predate this time period by almost a decade but many forget that playing cod in 2001 on a high end pc was a nerd thing, it seemed like blockbuster gaming to us nerds but normies didn't care until xbawks360 around ~2007+ which is when games got really mainstream and cinematic. I think the rot you pointed out has more to do with the advent of individualized marketing and people consuming content alone in their basement. When games were still retailed and cinemas where packed, products actually had to have a mass appeal, the ability to tailor everything to an addiction prone niche whale audience is what killed it past ~2014. Algrorythm driven game design turned everything into casino slot machine goy slop (everything is like 80s acrade machine shovelware again but looks like a movie) whereas before you needed cool box art, engaging everymen characters and story most people could relate to. Movie-like blockbuster games died at the same time actual blockbuster movies and cinema died - the triple A genre lived fast and died young.
RIP: Brutal legend, Bioshock, halo 1 & 2, metal gear solid, GTA, Red dead redemption, cod: modern warfare and witcher 3. And respect for trying: Ghosts of Tsushima (bland, no frills but played it straight), Cyperpunk(some
really cool moments in a sea of tumbler slop) and others...