Backwards compatability is very important. Most people, at least the honest ones who don't pirate their software, end up spending as much or more on software as they do for hardware. There's also the training and productivity aspects to consider when you talk about switching software, so changing to an incompatible system isn't a good idea for most people.

Multiprocessor systems currently exist and work very well if you're using an OS that supports SMP. Windows NT, 2000, and most versions of XP support SMP as well as some of the Linux versions. SMP is actually more efficient than running a single processor very fast because you can run multiple processes simultaneously without as much context switching. I used multiprocessor systems for a couple years and they worked very well.