You obviously have no idea how much these machines costed when they were released - especially fully loaded. Why should I not expect it to be supported longer, given the premium they demand on the Mac Pro machines?
Forced obsolescence of a perfectly capable, useful (and expensive) machine makes the Hackintosh camp a lot more appealing (mainly because of the apps I already have that are Mac only). Not to mention, the current Mac Pro's are very limited in how they can be customised after purchase.
The (1,1) and (2,1) Mac Pros were retired because they had 32-bit EFI and the new OS's needed 64-bit EFI....a technical limitation. Most of the Macs I've had that have lost support have been because of a technical limitation (be it RAM limits, 32-bit only processors, Power CPU's). This instance is just plain greed.
Microsoft obviously don't think the hardware is obsolete as Windows 10 runs flawlessly on mine in bootcamp for the Windows only stuff I do.