Then why does 91.123 have sections for both ATC clearances and instructions?
A bit of hair-splitting semantics on my part; ATC will give you a clearance, often with details (instructions) on how they expect you to execute it.
They aren't instructing you on the operation of the aircraft, and likely (though not always) aren't qualified to do so, anyway.
Hence "Unable" or "Cancelling IFR", in more extreme cases. Mostly, if you want something, (lower or higher, maybe) and they can't accommodate you right away, you work with 'em and are patient.
Sometimes, VFR, east bound over the Chesapeake Bay, they'll ask me to stay below 3K MSL, and in the spirit of cooperation, no problem, happy to do so. . .We help leach other out.
I remember a ground controller offering to give an instructor "taxi instructions" - the response was "What I want from you is a
clearance to taxi; I already know
how to taxi"
A little rough, but the point he was making for his student was, "you are solely responsible for, and the final authority as to, the operation of the aircraft." You don't get to blame the controller because you accepted a clearance that put you in jeopardy.
It doesn't come up much, 'cause the system works pretty well, and most controllers and pilots want to help each other out.
Just something to keep in your tool bag; a mind-set that the final call is yours.