The issues probably would not have been insurmountable; to presume they would've been so therefore that's why it wasn't worth attempting is just wrong.
The OP asked "Why didn't Q appear in any of the TNG films?" and I gave The Overlord
(and you) an answer, pulled together from numerous comments at numerous conventions.
You can think Paramount was "wrong" all you like, but Hollywood bean-counters don't think the same way as you.
It is extremely rare for a major motion picture script to be commissioned that will depend on one particular guest actor agreeing to take the part. The regular players of TNG had contracts with clauses to cover possible future projects. No guest actor is likely to sign up before a script is even written, and the studio is highly unlikely to greenlight such a project without a signature on a contract.
I never saw All Good Things as the perfect finale others have and apparently do.
de Lancie disagrees with you.