That's not a half bad idea. It wouldn't be any extra financial burden on Jagex to switch a world or two our way.
Perhaps if we only had one F2P world that's actually F2P, it being full all the time would be an incentive for people to become a member so they don't have to keep fighting to join that one world.
What they claim to be worried about (besides bots) is people cancelling their membership to go f2p. Sounds stupid, but some less active people might happily switch over during periods when they'd otherwise have kept membership.