The biggest problem with Apple releasing 7.6 and later is that Apple doesn't hold all the copyrights for those systems the way they do with earlier systems. Especially with 8.0 and above, Apple doesn't have total control. It may be possible for Apple to strip out the bits that they don't have copyright on, but I doubt they'd take the effort. Although it would be nice, even if they don't open-source it if they would at least release it the same way as 7.5 and earlier.