Windows 7 was not an expensive upgrade. In cost terms, it is equivalent to running the company car for an extra couple of years by replacing the worn tyres.
Like I said before, it isn't that simple and the license cost is the least of it.
To carry on with your car based example...
Imagine you have a fleet of 6,000 vehicles ranging from the boss' BMW to the gardener's lawnmower passing through 17 seater mini-buses, 3.5 tonne vans, and everything in between.
A decision is made to extend the life of these vehicles by fitting new tyres. So an order is made for 30,000 new 18" low profile tyres (coz thats what fits the BMW and every vehicle must have the latest and greatest. And a spare).
What you end up with is one BMW with five new tyres, 29,995 useless tyres, and a fleet manager out of a job. Not to mention the time wasted calling all those vehicles in to have the tyres fitted. And another project to get the right tyres for the right motor.
No-one in their right mind takes on a project of that magnitude without doing the proper research and planning first.
Going back to window 7. Lets take the same number of upgrades: 6000. Lets assume that every upgrade will go perfectly, each PC has only one user and there's no knock on effect from a PC being out of use. Lets be generous and say each upgrade will take 2 hours start to finish. I'll even go as far as allowing all the PCs to be identical.
Also assume an 8 hour workday excluding breaks. So four upgrades a day per technician.
That's 12,000 man hours to perform the upgrades with another 12,000 lost to the required PC downtime.
Now, the real world isn't perfect. You can bet that all the PCs aren't identical, that some piece of software won't run properly or some piece of hardware doesn't have a driver (moving from 32bit to 64bit? better check all those drivers exist and are signed). Which adds to the time and cost.
And that's ignoring any staff training. Moving from XP/vista/7 to Win8? Better get some trainers in.
In car terms that would be replacing 6000 automatics with 6000 manual transmissions. Your drivers would need retraining and new driving tests/licenses.
Swapping new hardware for old isn't actually much easier. Someone still has to configure it and install it at the point of use.
As for backwards compatibility, you're joking, right? I've lost count of the times I've had to bin perfectly functional peripherals because there's no driver for them on the latest/greatest version of windows. And software, though not quite as frequently. (yeah, with software you might be able to run it in a VM but that's more complexity and cost)
Once again, if you only have one PC, it's easy. You can afford to upgrade first and fix later. When you have 6000 you can't afford to work that way and the license cost is only a small part of it.
(Why 6000? One of the affected NHS trusts said that's how many PCs they have in an interview with BBC news)
Attempts to contact me outside of thes forums will be ignored unless signed in triplicate, sent in, sent back, queried, lost, found, subjected to public enquiry, lost again, and finally buried in soft peat for three months and recycled as firelighters