Why would Microsoft want to divide their resources supporting old operating systems? The sooner they are able to shift more people onto 10, the sooner they can assign more people and resources to improving and securing it.
I don't really care what Microsoft wants. Allocation of support resources is a Microsoft problem, not an end user problem. Chrysler might want to sell new cars and tell you that you just have to get the newest model because "it's totes better than the old one" but does not remove their obligation to fulfill the promised warranty.
If Microsoft doesn't want to "divide their resources" supporting prior versions of Windows, then they should stop promising such long periods of "supported until" windows. This really shouldn't be a hardthink concept.