By definition, it cannot be physically possible for another a/c to be able to land on the same or crossing runway at the same time. I struggle to understand how such a clearance can be given to more than one a/c simultaneously. The only exception is a "land after"; & that can only be given to one other a/c AFTER the first a/c HAS landed.
Perhaps your definition doesn't allow, ours does. By the time I cross threshold, the previous aircraft is off or the appropriate separation exists. If not, then the controller sends the aircraft around/missed approach.
It works just fine. What it is, is different than what many of your countrymen are used to, therefore it's wrong.
The system hasn't failed me so far in many, many thousands of hours flying to 7 of the 10 busiest airports in the world.When I've crossed threshold, all was good. A few times it wasn't, the controller took action. Now without doubt, you're about to hear (likely apocryphal) stories otherwise. Like all systems, its not perfect, but it works well.
My preference is to get the clearance early and not on short final possibly. Others may prefer otherwise. Doesn't make one right and the other wrong.