The main reason to hold on to older systems IS Windows 10. It might be new, but comes with so many needless changes that productivity will take a nosedive when it is rolled out. Even simple things such as changing default applications take a multitude of steps under Win 10, just to be redone after each update. Aside from moderate improvements in security, Win 10 has absolutely nothing compelling to offer to the general consumer and enterprise user. Some might like the UI (although I have no idea why), but it won't help them get more things done. Worse even, with Win 10's dismal hardware support a lot of accessories need to be replaced driving up cost. Win 10 also gives the user much less control and decides on its own when it is a good time to reboot the whole system, even if that means processes being interrupted. Just because a user is not interacting with a system does not mean that the system is not used.
Sadly, the alternatives aren't much better. Support for Win 7 and 8 will be cut off in a few years, cannot buy licenses anymore (except for MSDN), Apple lost its mojo while keeping it excessively high prices, and Linux is too fragmented and open source projects are often run by egotistic developers who do their job purely for self-fulfillment, but not to serve users. Right now, there is not a single OS ecosystem worth buying into.