According to Microsoft's own numbers, there are nearly 400 million XP and Vista machines in the world today and 500 million Windows 7 & 8 machines. If you look at business users specifically, the XP/Vista numbers actually exceed W7/8. There's good reason for that: if you rely on a workstation in your business, you stabilize it, standardize it and then leave it the flick alone.
I can only imagine what will happen when every business application vendor announces that no updates are going to be available to those 400 million users. Fortunately, most vendors aren't bound by the Windows Logo program, which lets Microsoft dictate what they can and cannot do.
I am not convinced that there is any technical reason for being unable to distribute an installer that's Vista/XP compatible, other than Microsoft is forcing vendors to abandon those operating systems in order to spur sales. Yes, I know that critical pieces of a SONAR installation are actually sourced from Microsoft and have to be distributed unmodified.