If the main use case is torrents, I think the reason why the method you mention isn't commonly used is that it is complicated to understand/set up and hard to

If the main use case is torrents, I think the reason why the method you mention isn't commonly used is that it is complicated to understand/set up and hard to verify. I've seen more advanced users encapsulate everything in a VM so that non-VPN traffic can be blocked globally by the OS.