Some bad assumptions in this article.
We are running an estate of HP DC7600 and other HP PCs many of which are "significantly" old. We are not losing thousands in productivity. We are not running risks with security. We are not running out of date OS. Just because some of these shipped with Windows 2000 or XP doesn't mean we are still running that. We have Windows 7 and Windows Vista (I would prefer all W7 but we can't do the licencing). Why Vista you ask? Because some WS2008 functions require a fellow "Longhorn family" OS in order to run and we had a business case for those functions.
We have very few hardware failures. Our aging PCs run at an acceptable pace for the work which they are asked to do. Most of our serious/business critical apps are web published not client server so as long as they can run a browser they can run. Shaving 30 seconds or even 2 minutes off the boot up time on a Monday morning won't save a lot. We run modern monitors, mostly. I have seen some people offering some really over cooked arguments about power saving but all of the ones we checked the maths on they didn't add up at all. People make bad assumptions then publish online and other people believe what they read and make more bad assumptions on those bad figures. Using Vista and W7 gives you a bit better control over power management but its still not "right", and thats the case on old and new PCs (oddly enough the PC on my desk is a brand new one, who'd have thought that?)
We have our patching sorted, we have our general updating sorted, we have our monitoring sorted, a tried and tested set of group policies. We have very few problems of OS reliability or application failure, even when running Vista on 4 and 5 year old PCs.
If you run a business where everyone needs to run heavy duty applications on their PC then you won't get by using this approach. But as we are looking to the cloud more and more (public or private) the hardware on the desk of Mr or Mrs Averageuser is less a critical factor. So long as you spec it right, keep it patched, keep the crapware off it, etc.
Look at the cost of HW refresh? Blimey.
This article reads like the argument a chap I worked with made. He swapped his 5 year old Astra for a new Focus and was crowing about how he'd save £50 a month with the better fuel consumption, ignoring the £175 a month extra it was costing him in finance to buy the blessed thing...
I'll ditch hardware when it breaks and can't be fixed economically or when it is no longer supported by or capable of running the SW/OS I need to run on it. If you have Vista or W7 deployed then multiple HW generations or even vendors is no big deal, you can still have unified build images and keep your estate easy to manage.
You don't have to rip and replace.