Ditching all the legacy stuff in your CPU is nice and easy. Then you just have the task of persuading customers to buy your new architecture and software developers to develop it, like Intel did so effectively with the Itanium systems we all use now, wiping out those silly backwards-compatible 64-bit chips from AMD ... whoops.
There is a market for nice clean efficient CPU designs: anywhere you don't need to worry about existing code bases, essentially. Does my current washing machine use the same control chip as the previous one? Who cares - if the manufacturer wanted to switch from, say, Z80 to 6502, that's up to them. If my new laptop couldn't run existing x86 code, though? It would be most of the way to paperweight status already. Ask anyone with an iPhone or Android phone to think about tablets: buy a different platform, re-purchase all their apps and start again, or keep using all the familiar apps they've already got and paid for?
It can be a pain sometimes - but if you couldn't be sure that shiny new PC would run Windows XP happily, or that Windows 7 would run Office 2003, your favourite old shoot-em-up, that accounts package you rely on and the free PDF generator you've got set up to do stuff just the way you like, you won't be at all happy. Sure, you could upgrade Office - but then some of those macros you use for client invoices break...