I think it's backwards to think a Q movie could not have been done because DeLancie would say no, or demand too much because he's the only one who can play Q.
I'm sure he'd want an agreeable script, and compensation. The challenge would've been to come up with the script, and for the parties involved to agree to his pay. 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.
Of course, we all would want DeLancie for Q.
I never saw All Good Things as the perfect finale others have and apparently do. It was just another time travel story to me. But a thread was left with Q's "see you out there" comment. And I doubt Q would go away.
Of course, the story would have to advance Q, advance the relationship in some way. We're not going to see the same Q we saw in the series. And there would have to be some other story that Q was or became involved with.
And of course, any growth or change to the Q character is likely to piss off some portion of the fanbase, just as the Borg Queen did in First Contact.