Steven P. McNicoll wrote:
Pilots should understand that the alternatives to accepting the new
clearance are probably a very lengthy reroute, diversion to another
airport, or IFR cancellation.

Those are certainly possibilities, but they're not the only ones. It
seems like I have the following conversation with NY Departure about
once a year:

"Proceed direct Sparta, climb and maintain 8000"

"Unable 8000 due to icing"

"Why didn't you tell the tower this?"

"I did. I filed for 6000, JFK, V1, but this is what I got. I told
clearance delivery that I'd be unable to get that high. They told me
to work it out with you after I took off".

"OK, tell you what, maintain 6000 for now, fly heading 180, direct JFK
when able, I'll have the rest of the route for you shortly"

The point here is not that I was able to force ATC to give me what I
wanted (I can't "force", only negotiate), but that I stood my ground
and refused to accept what was unsafe for me.

And yes, I understand that the possibility exists that there will be
nowhere for me to go except back on the ground. That doesn't mean I
can't explore other options with the controller. And if I ever paint
myself into such a corner such that I need to declare an emergency to
get a clearance I can accept, then I've done something wrong long
before it got to that point.