That is the BIG Question!! WHO? The USCCB has ignored many directives from the Vatican in the past. They will ignore this one also as I believe it will be self incriminating for many of them.
The WHO is the Laiety, the People of the Catholic Religion themselves. If Rome and especially those around the Pope are serious about enforcing the directives, then the Laiety will have somewhere to go. Otherwise the Church will remain in the state of destruction that is has become since Vatican II
Why does the church not allow married men to become priest?? I heard that 100-200 years ago the church knew if priest were married once that priest died they would have to continue supporting the spouse. i.e. that not allowing priest to marry was not some devine intervention from God but based soley on ecomonic issues.