Goldfish Watcher;
The reason that a pilot wouldn't comply with the old restriction is because the restriction no longer applies. If you need them to be level at a certain place, then you MUST advise them via an updated clearance as to where you need them to be for separation purposes.
We expect pilots to tell us when they can't do something. We don't read their minds. The same goes for us. If we need something then we have to issue an appropriate clearance.
regards