Doesn't this diminish the value of a startup, say, if the buyer hands the principals a mountain of cash in a buyout then the principals use that cash to build in the same industry / market the startup they would have if they had all that cash in the first place? Without having the non-compete with the principals, who would even want to buy your startup?
I don't know if you've noticed, but there's this little state "California" that gets a lot of press in the start-up scene. Non-competes are basically illegal there, and it doesn't seem to have hampered either the start-ups, nor the build up of megacorporations who buy them.