Well this seems to be the crux of the issue. Where do you get the impression that this is possible?
I’ve spoken with numerous OS X and iOS developers, and they’ve all complained about the same issue: the early beta releases are not good indicators of what will and will not break, so they can’t prepare for GM based on those. Fixing things that break during the beta cycle may be completely wasted work, since Apple may fix it themselves at any point. Alternatively, things that seemed to work earlier in the beta cycle will break later, even only at GM. Some issues are in core OS services where Steinberg “fixing” it would mean creating a separate code path just for that OS release where they kludge around that issue. That is not healthy software development.
I’m sure many of us think of Steinberg as a big company, because it’s one of the major DAW houses and Yamaha is their parent company, but the truth is this is a niche market and the companies that work within it really have to manage their limited resources. Throw into that the fact that realtime audio applications are one of the most challenging to maintain through OS updates, and there’s no scenario where this works 100% of the time. Ever. I would love compatibility on day one also, but Steinberg can’t play whack-a-mole with beta APIs, and Apple isn’t going to deliver GM months before release.