The system you are talking about is like UO has, which I'm not saying wont work, I'm just not seeing the need to lock or unlock them. If you want to keep a skill high or near max, it should be your main focus to keep it there, and with a little effort (using the skill once or twice a day) you can keep it from decaying.
To me it goes more to the point of instead of people having to choose what skills they want, the play style chooses it for them.