Now, show me where this is stated in the AIM, CARs, or Instrument Procedures Manual. I can find no record of this ATC expectation on the pilot side of things.
I informed TC of as much, and they promised an AIP amendment as a result. That was two years ago, and unless I've missed the change, I'll consider the matter unresolved...
I'm not surprised it's not specifically covered in CARS, or any other document. I don't think it need be.
Bottom line is, you as a pilot are given a clearance, (hold, heading, altitude, whatever). If you accept that clearance to must comply. If you can't comply you don't accept the clearance, and its up to the controller to come up with something else.
If you cannot meet an altitude requirement as specified, don't accept the clearance. If after accepting the clearance, time passes and you realise you can't comply, you certainly can't charge ahead in contravention of an accepted clearance. I'm pretty sure you'll find something like that in CARS.