The PCV valve is not the only opening into the crankcase for blowby gasses to escape through. Theres also the air inlet pipe or "engine vent pipe" that supplies the fresh filtered air into the crankcase where it does the job of scavenging any blowby gasses escaping past the rings. Any gasses etc that are scavenged will then get sucked up through the PCV and then fed into the inlet manifold where they are then consumed in the combustion process.
The workshop manuals also say that if the blowby gasses exceed the maximum volume that the PCV can handle, either because of a worn engine, or when the engine is running under extremely heavy loads, then the blowby, as well as being sucked in through the PCV , will also be vented back through the engine vent pipe that usually supplies the fresh air into the crankcase, it can then be drawn into the engines intake air where it can consumed in the combustion process.
Years ago, I used to see a lot of engines that had a pipe that just vented the crankcase fumes to the atmosphere, which was obviously not good for the enviroment, so they came up with the idea of sealing up the crankcase, introducing fresh air into the crankcase and then drawing it out again through the PCV valve to keep a constant flow of frech air through the engines crankcase for scavenging purposes.
If a crankcase was completely closed off to the atmosphere with no PCV system working, then the engine will most likely build up too much positive crankcase pressure and blow out an oil seal.