I think at some point they have to say enough is enough and start fresh, like Apple did.
Which is unlikely to happen: Microsoft lost the mindshare of too many people over the years. the main reason for their domineering market share is because of lock-in. You use MS because you have applications that require it.
Now, if Microsoft scraps their entire product and comes up with an brand spanking new one that is not backwards compatible with all their tangled mess of old API's, there is no legacy software holding you back, and no more lock-in. a completely new MS OS without backwards compatibility means a great opportunity for people to jump ship to Apple or Linux as well -- After all, they're starting anew there as well. MS will do everything in their power to keep people locked in. (legal as well as not-so-legal as numerous court cases clearly display)
On the same line: Intel designed their Itanium line of chips -- a brand new 64-bit system, without 32-bit backwards compatibility. Arguably superior to AMD's x64 extensions which DID allow old 32-bit programs to run as well.
The end result? Intel wasting a billion dollars on their itanium line, and quietly implementing AMD's x64 extensions in their Pentium-D and Core 2 lines of chips.
Backwards compatibility truly is both a blessing and a curse at the same time.