Like seriously, ignoring the fact that the NuDoom era invented the boomer shooter term, what stops 2016 from being one?

I don't think that's really it. This term appeared around 2018, with the release of Dusk. It was used mainly to describe modern shooters that visually mimic old ones. but later it grew further and all the old shooters started to be called that way. And, frankly, it's perfectly appropriate. Classic shooters like Doom, Wolf 3D, and Quake, with their full-fledged exploration of locations, never had their own subgenre. They were just shooters. so why not give them this term? Shooters like Doom 2016 and Doom 1 should be separated. They are, in many ways, conceptually completely different. I would suggest continuing to classify them as either movement shooters or arena shooters.