surely more will get released as and when people don't make payment within the time period though?
I seriously doubt that. I think this is the third time I tried the UEFA ballot and not once did I ever hear anything about what happens to the tickets that aren't paid for in time. I would imagine they somehow disappear and get used by the UEFA family or whatever.
could easily have waited until Thursday until we know the finalists
That's wouldn't have made a difference though. You couldn't delete your application, so the people in it would have been the same number as on Thursday and you have to pay for your ticket by Friday, so that would have been way too close.