Just like you wouldn't play on servers without mods, I believe people wouldn't play on servers that allow wallhacking without the fear of being banned by VAC.
There are probably far more wallhackers than the ones dumb enough to trace people behind walls or invisible spies. These wallhacks are really easy to install and the 2 mentioned here have over 15,000 downloads.
If you want to see how easy it is to use.
http://www.filedropper.com/mathackv6
Just unpack into custom folder as is.
Start a server
sv_cheats 1
mat_reloadallmaterials
sv_cheats 0
disconnect
After that it'll load on any sv_pure 0 server.
Votebans have always been global and it doesn't solve the problem. When the whole server decides someone has been wallhacking, the damage has already been done. But really, the best solution would be to ask valve to disable ignorez if the original material doesn't have it. You can still use mods on our non-vanilla servers.
So far we haven't noticed a large drop in population so we are probably going to continue with this.