I can be cleared to fly along an airway when there is 'something' on it in front of me, and so I should be able to be cleared to land on a runway when there is 'something' on it in front of me. Happens all the time, other parts of the world obviously, but controllers are quite capable of safely applying separation on the ground as well as in the air.
Landing clearances can be cancelled and I can be told to go around. As we all know from venturing into some of the more hallowed places on this site, a go around is no big deal
