In our home office, the leftover of a small business venture, we're still running XP on a digital music server, Win7 on a digital picture server, and Win 10 on two desktops that we use for the more critical records and software like my SQL Server . My thought has always been to stay up to date - SORT OF.
There's a delicate balance to what is critical, both content and potential downtime, and what is not. Coincidentally, while I was writing that last sentence, the XP music server went bonkers, just hung with a seriously bad noise. So far, it is rebooting. This machine has also lost its wireless connection and is waiting for me to locate a network board in my parts storage.
So to me it boils down to how critical the machine and OS are, versus the cost and risk of upgrades just to stay current with support. And of course, how exposed the system is to security threats.
Disaster Recovery = Backup ( Backup ( Your Backup ) )