To be fair -- it's been a couple of years since I played Valve multiplayer games regularly. Has cheating increased at a high rate in the past, say, 3-4 years?

I will say, I think the biggest problem contributing to high levels of cheating is the move away from dedicated servers. I actually find that VAC is historically very effective at catching cheaters, but with a necessary lag between detection and bans -- and that lag used to be handled by server admins.