joel spolsky wrote an excellent article how microsoft shifted from making sure all applications continue to run on new os releases to rewriting everything.
Raymond Chen writes, “I get particularly furious when people accuse Microsoft of maliciously breaking applications during OS upgrades. If any application failed to run on Windows 95, I took it as a personal failure. I spent many sleepless nights fixing bugs in third-party programs just so they could keep running on Windows 95.”
Inside Microsoft, the MSDN Magazine Camp has won the battle. The first big win was making Visual Basic.NET not backwards-compatible with VB 6.0. This was literally the first time in living memory that when you bought an upgrade to a Microsoft product, your old data (i.e. the code you had written in VB6) could not be imported perfectly and silently. It was the first time a Microsoft upgrade did not respect the work that users did using the previous version of a product.
he goes on to suggest that microsoft is trying to save the rich client by all means necessary:
There’s no way Microsoft is going to allow DHTML to get any better than it already is: it’s just too dangerous to their core business, the rich client. The big meme at Microsoft these days is: “Microsoft is betting the company on the rich client.” You’ll see that somewhere in every slide presentation about Longhorn.