The problem Adam has isn't about dropping support for past technologies, it is just the word at the moment seems to be pretty clear that Avalon will be for Longhorn only. There are always these rough transition periods in the march forward (DOS vs Windows, Win16 vs Win32). Seeing as how we are still looking so far into the future it is hard to picture what the migration plan will look like.
The difference between the transition between DOS and Windows is that they are completely different - DOS being mostly text based, and Windows being GUI based. It would also prove to be a lot more expensive as well.
Also, there are far more Windows users than there were DOS users, and there is much less incentive to move to Longhorn - i.e. how would it benefit people who do just Word Processing and email?
What I see in the future is Microsoft having a much smaller market share. If you want basic feature, stick with what you've got (2000/XP) or go with someone else (Sun, Novell, Redhat, IBM). However, if you want the latest (Microsoft) bells and whistles, go with Longhorn (although other OS's will still have compelling features).
It is going to be a lot harder to sell Longhorn than it was to sell Windows 3.1/95. Also, there are people switching to Linux (which will unlikely switch back to Windows). Even moving to .NET is not always the best choice (except perhaps for ASP.NET) - if you have programs written in VB6, why rewrite them in .NET, what benefits will users of that program get? If there was a tool that could convert a VB6 project into a VB.NET project, maybe people will migrate (even then, the .NET runtime is required - VB6 runtime is a lot smaller, and is already installed on most PC's).
Microsoft will find it hard to keep its existing customers, it will also prove near impossible to get Linux users to switch back (as they have far more options - having problems with Sun? Go with Novell/Redhat/IBM).
Do not underestimate the appeal of Linux - it is here to say (and Microsoft can't really embrace it, as it will require a big change in strategy).