In fact, I believe they are doing exactly this in El Capitan. Under the umbrella of "Rootless", you will no longer be able to arbitrarily modify [1] some special blessed OS processes, like Finder.app.
Some discussion of the politics of this issue: http://atp.fm/episodes/128
[1] haven't read enough to know the exact mechanism, but presumably restricting some forms of IPC like Mach messages?
And it's not unlikely that Apple will break them at any time. Apple won't even go through the trouble of making an announcement when they do so.