If you can't get to the QB, then dropping more in coverage makes those WR's less open and gives them additional coverage help.

It isn't all or nothing. Blitzing (hopefully) reduces the time the QB has to make a throw. But dropping more in coverage (hopefully) reduces the throwing windows and open receivers. Both can be successful. Most successful is probably mixing it up to keep the QB uncomfortable and not knowing if he has to decide and throw quick, or take his time to make sure he's throwing to someone who is open.