Wouldn't it be a lot easier for the non-uploaders to, you know, just pull the plug? Or change the input to the digital existances in a way to simulate a reality "outside"?
No, because – as I mentioned above – the uploaders would still maintaining all kinds of technology to monitor and control the outside world in order to ward off any threats (natural disasters, sabotage from the non-uploaders) to the computing infrastructure on which the simulations are running. If the majority of society chooses to upload, then that suggests that such protective infrastructure is already so stable and advanced, that the non-uploaders would be powerless to simply "pull the plug".
You're getting very close to ending up with the script to The Matrix.