Well they could go with some other methods such as pay per export, or pay per game slot, but those would have some have some negative cavates such as a premiums for having to develop it, and store project files on their server.
As I recall a lot of people got their ******* in a bunch over the idea that they can't do anything with project files without the editor, so not having access to your own source files would probably tie them into a knot, but thats how that kind of plan would have to work.