^You're not comparing like with like. SG & Friends had definitive casts who the audience wanted to see together. By its very nature, DW rotates and changes cast, from lead onwards. There is no need to worry about the incumbent actor wanting a pay rise when there's a built-in in-universe explanation for replacing him. As regards the budget thing, a DW film is still going to have to be funded by the BBC, which has a finite budget. Should they spend the equivalent of an entire season's budget on a 2 hour film just so that we can see cooler SFX on the big screen?
I can't see a Doctor Who theater movie being finded by the BBC. And a movie would cost far more than the budget for an entire season.
But when you bring in a new Doctor, they are the new face of the Doctor Who franchise. It is quite a disservice bordering on insulting to them to bring their predecessor back in a movie. Besides, much as this phrase is not liked due to being wrongly used, when Star Trek did TNG movies while DS9 and Voyager were in production, it lead to franchise fatigue, which resulted in the eventual downfall of the franchise. Stargate was also heading in that direction. The best model for a franchise is to focus on doing TV shows or movies, not both.
So who would fund it? And how much would they spend on a film, whose fans can see much the same thing at home for free?
Whomever would get the films rights would fund and produce the movie. And yeah there's little reason to make a movie when you can see it for free on TV. Like I said it'd be better to make the movies after the series has ended.
Personally, I'd prefer that the tv series renews and reinvents itself and continues indefinitely than end it and go to the cinema.
Interestingly in 1986 during the show's troubled late era, Michael Grade and Sydney Newman (DW's co-creator) considered bringing back Patrick Troughton as the Doctor. However nothing really happened with this, and Troughton died around that time. Some fans have speculated that the Troughton idea influenced the casting of Sylvestor McCoy, who bears a physical resemblance of sorts to Troughton. (Although they are very, very different actors).
I like Capaldi but I wouldn't particularly want to see his Doctor on the big screen. Get as much of a "big screen" actor as they can find and go big big big. Maybe Tennant could get away with it but Capaldi and his Twelve works better with the intimacy of the living room in my mind. I would treat TV and film as different beasts with different strengths. TV Doctors only last three seasons anyway, they'll manage.