Actually, it would. It's relatively easy to move forward from iOS6->7 code, since at that point you're more or less able to tighten up and remove a bunch of code. Not quite so easy to go back, since then you have to a) stick guards around potentially optional usage of iOS7-only APIs (as well as find them, since all apps have to be built using the most current SDK, so the IDE is no help here); b) not base any functions on any iOS7-only API additions or improvements (CFNetwork improvements, etc.); c) rely on deprecated properties, and hope they don't get removed in the next major release.
Not insurmountable, but not completely trivial.