I take it you’re not a software developer? (I am). You have a confused notion of what testers and developers do. When you have a new beta developer release out for an OS, and an existing program, testers and QA engineers should (and do) test whether the program works with the new OS, create bug reports for any issues found, etc.
You seem to think that testing is only about making sure existing versions of a software work in the stable version of an OS. That’s not the case in any major software company. Some smaller shops might do it that way merely out of lack of available resources, but most competent small shops will too test their products with the developer betas to fix whatever needs to be fixed.
That’s how all those companies are able to launch updates to fit a new OS on day one or within a couple of weeks.
The number of changes doesn’t matter at all, since only the ones affecting your product are important. The total number of changes is across all subsystems and programs, including ones not touched by your product at all. E.g. thousands of them would be to the revamped Web engine, etc – and have nothing to with Cubase.
And of course there’s always the GM, released weeks before the new OS, that has few, and usually NONE at all, differences to the final version.
The reason SOME enterprises take “up to two years” to implement an OS upgrade is because enterprises are slow moving behemoths, with tons of legacy crappy code, even stuff depending on IE6, Active X and what have you.