I work in sound actually (pro tools, film sound post production), and use tons of plug ins, so I'm very used to manipulating sound.
Many here have made your suggestion but it's highly unlikely an audiophile inclined towards tubes would adopt it. First, such VSTs/plug-ins are typically used for professional work integrated in to DAWs. Nobody that I know of has made a tube emulator plug in for easy integration in to a typical stereo system.
Also, even a plug in may not emulate precisely what someone likes in their particular tube amp.
Finally, your suggestion misses a big part of the appeal which, as mentioned earlier, is the physical tube amps themselves. Many find them quite pleasing both conceptually and aesthetically, as I do. They look beautiful when they are on glowing, and there's a satisfying "connection to audio history" in using such older style gear. That's part of the fun. It would be like saying "hey, you don't need a turntable or records, just get a VST to add surface noise and crackle to your digital music."