I can guarantee you they could split the changes and keep the current stuff for PvP and push the patch live, they have tech in the game to do this already.You'd be surprised. Writing code is just 1 part of it. You've got to account for the time it takes to run a build, deploy to staging, test that it works, deploy to production, and then test again. It's no small feat. The build process alone can range from a few minutes for small pieces of software, right up to taking several hours to complete. There are huge systems in place that ensure everything goes as planned, and if something goes wrong, it is stopped before it's too late. Even doing this for relatively small pieces of software requires that you never break these systems.
When you're adjusting core systems, such as player level, and ability power and mechanics, you do not want to have to also simultaneously maintain a separate, nearly identical set of code, just so one group of players can continue using the old mechanics in one part of the game. It is only feasible to update all these systems at the same time.
With corruptions having played such a huge part in this season, taking them away for a few weeks near the end would cause such a huge uproar from people. They can be the difference between getting the gladiator mount or being pushed down. Likewise, with so many classes and specs getting huge overhauls, it would not be reasonable to expect people to 'deal with it' because their previously strong spec got nerfed to the ground. And god forbid there are any major bugs that would break a spec entirely.