I think you’re missing the point of why they’re buying cheap game keys. In fact, it sounds like you think a ‘ban’ is something bad to these players or will stop them. If it did, I’d probably be enjoying Rust still.
Not even VAC bans are perfect, although it typically stops the poor unfortunate kids who truly don’t know better at least.
Minecraft anticheat won’t be perfect either. It is a necessary and functional safe guard as is usually the case with anticheat (minus rootkits, fuck those useless tools), but people will always slip through. Note what other people in this thread are saying.