It kind of works like that already. After you reach a certain level in a skill you don't get any new skills or abilities after that. But you still can gain levels. If you get p2p, you can use those levels, you've gained in f2p. There are more f2ps than p2ps. -> More f2p worlds than p2p worlds. If there are not enough f2p worlds, people can't play because all worlds are full and there will be less members. Simple as that.