Great, you have taken one step - ie you know what doesn't work - now go out and find what does.
He has.
XP works right AND meets his customers' needs. Why would he change that?
So if you put a working Chevy engine in your Ford and it doesn't work you will go to a car forum and jump up and down and complain that the Chevy engine is crap? (knowing my luck and having little car knowledge this would probably work ok, but I'm assuming its not. If it does then substitute a Toyata engine or something).
No, your analogy is pretty good, just for reasons that you don't understand.
You SHOULD be able to replace any engine with any other engine of approximately the right size, on any vehicle - whether that means XP Pro or Ubuntu 7.10 or Windows 2000 or Windows Vista, any desktop OS that you meet the basic hardware requirements for should work in your PC. Sure, server OSen probably won't work right, but you didn't buy a truck. You bought a car.
What happens, however, is that when you stick Vista into a PC, what you get is the equivalent, in your car analogy that you don't understand, of tossing in an underpowered 4-cylinder sewing machine engine into your brand new F-350. Sure, you CAN do it, and sure, if you're lucky, almost everything will work just fine - but you'll be slow and noisy and wasteful.
I'm pushing whats best for the client and finding out their needs and wants.
Funny how that turns out to be "XP is better" in just about every case, isn't it?
If they want Vista I sell them a working Vista machine.
As do I. However, I make sure I'm selling them a version of Vista that carries downgrade rights for *when*, not if, Vista fails to meet their needs, and if their hardware doesn't have XP support, I make sure they know that before they order it.
when in the future you can't source XP any more) what are you going to do? By your own comments you will not sell Vista so will you be selling just Linux machines and or 2nd had copies of XP?
Vista Business or Ultimate = XP Pro. One for one, right out of the box. Owning a VB or VU license gives you the right to use XP, just like owning XP Pro gives you the right to install 2000 instead.
This is not complicated.
And by the time ME2K7's replacement comes out, either "the year of Linux on the desktop" will finally have arrived, or MS will have fixed the things that make a *working* installation of Vista far less attractive to the consumer and the technician than the XP Pro license they replaced it with.