This debate could go in circles forever. W3D is old, inferior to UDK and full of holes. We all know it, but as said before we like it. It's not an unhealthy attachment, we just like it. Sometimes things just have a certain charm to them, regardless of age or functionality.
It comes down to opinion, and that's something that can't be disputed.
RE backwards compatibility; Network Protocols haven't changed in years and i very much doubt they will. Even if they did, it wouldn't be hard to build an in-between client to make it work. Also, There will always be middleware solutions that enable backwards compatbility. (virtualXP for example). If we're all still interested in holding onto W3D in distant future, i'm almost certain it will be possible one way or another.