enthrowplumI think this is already a good enough reason for many people to download an additional mod (assuming the anti-cheat is of sufficient quality). But maybe I'm being too optimistic?
I definitely think its a good enough reason! The issue is the enormous dev effort.
You're stripping VAC from the equation and your solution needs to be much, much better before its worth it for leagues to even think about switching. There are people in the community with the skills and knowledge to do this pretty well it would just be a major lift. In fact my understanding is that people like steph have done some really cool anticheat work on similar standalone TF2 mods (not sure which one off the top of my head, but I've heard good things). The tech is possible, its just a wild amount of ongoing effort by people with very specialized knowledge and limited time. This is especially the case as leagues/ pugs switch over and the cat and mouse game with the cheat devs ramps up.
What about megascatterbomb and his development through demos in matches? If not, why?