There is a very specific security threat when a single entity controls 50% of the networks hashing power.
50% isn't actually a hard limit. It is possible to do the same attack with 40% or even less, it will just succeed less often.
Ok, so the risk is still there, and needs to be addressed. No firm limit, check. Either group action or voluntary responsibility will work. Ok.
So who's it gonna be? the miners? tycho? Or the (some) miners setting up tycho for the chain?
All it takes is pop-a-box. Or corruption...
But we don't have any of that lolololololol