How could a dev theoretically stop their game from getting datamined?

Can't you just encrypt files?

If you're able to hook and sniff the executable whilst it's running, you should be able to reverse engineer the calls used to decrypt content if they were sloppy about it.
Tough to crack cryptography is often avoided or used sparingly because it does slow everything do, loading times, performance, etc.
Is why Denuvo fucks everything up.