I think there should be a minimum, but you give 500 norm wins too much credit - its very possible to fail to learn even the basics whether you're playing norm or not. I spend quite a bit of time relaxing against ai, and it amazes me how weak some 30s are at so many key fundamentals.
Don't get me wrong, I'm far from impressive in my own right, but relying on 500 wins to improve player abilities is a bit of a fallacy considering even the worst players can accumulate 500 wins through sheer attrition.
Implementing a minimum requirement is a great idea, and one I'm sure has come up in the past. But the difference between 100-250-500 isn't going to be that drastic.