It's still rare that a band exclusively handles the soundtrack and score to a movie. But when it happens it can be magical. It would be difficult to imagine either Highlander or Flash Gordon without Queen. I would dare to say that both of these movies legacies would not exist without the music. Lets put this in a contemporary context. Could you even imagine Tron Legacy without the music of Daft Punk?
Yes, Flash Gordon is a little cheesy. But it is the good kind of cheese. And the music of Queen had an epic feel that enhanced the movie and made it a sci-fi classic. The same epic music helped Highlander become another sci-fi classic. A current attempt to remake Highlander seems to be falling apart again and as far as I know (really hope I'm not jinxing this) there are no plans for a Flash Gordon reboot. I think it would be difficult to reboot either without people missing Queen in the mix somewhere. It is possible to go without and still have good films. Anything is possible after all. But there would be high soundtrack and score expectations. Besides, when the originals are this good, there is no need to try and replicate the magic.