We're not changing the meaning of yield or generator.
We're just making it *possible* to use yield(-from) and generators as coroutines; that's actually a long path that started with PEP 342. No freedom is taken away by PEP 380; it just adds the possibility to do it without managing an explicit stack of coroutine calls in the scheduler.
If we believed that there was no advantage to spelling a blocking call as "yield from foo()", we would just spell it as "foo()" and somehow make it work.
But (and even Christian Tismer agrees) there is a problem with the shorter spelling -- you lose track of which calls may cause a task-switch. Using yield-from (or yield, for that matter) for this purpose ensures that all callers in the call chain have to explicitly mark the suspension points, and this serves as a useful reminder that after resumption, the world may look differently, because other tasks may have run in the mean time.