There is enough evidence to say it isn't a theory. Clip after clip after clip of weird shit keeps popping up. There clips where someone gets killed in 2 melee hits after getting 4+ melee hits on the other guy. Or people mag dumping someone only to get 2 shot. There was also talk about aim assist just turning off randomly mid match if you were doing too good. People have reported weird issues with latency where they will do really good and then suddenly they get massive packet loss until they die a few times or someone even better than them joins the match and then the packet loss vanishes. How many coincidences do we need before it stops being a coincidence? There is a reason MWII went from 500k players to 100k within a few weeks on steam. Games in general just feel more and more like shit as time passes.
Not to mention all the other stuff not related to SBMM. The UI sucks, cut scenes you can't skip when you open the game, garbage battle pass, missing game modes, shit maps like border crossing, the slow phasing out of K/D and such, the shitty bundles.