After Windows 10 was released, Microsoft started backporting most of its telemetry capabilities to Windows 8 and 7 and turning them on by default, rendering those versions just as "backdoored" as 10. That was what pushed me to accept that Windows was never going to get better in that regard, and so I've upgraded all my Windows 7 and 8 machines (except my Surface RT obviously) to take advantage of the new features. I'm especially enjoying native NVMe support on my new workstation build; I can't go back to a spinning HDD without feeling like I've stepped 20 years into the past.
All that said, I agree with your last point about upgrading being a fair compromise compared to staying in Windows 7 or gasp Windows 8.
Are you sure? I did a lot of Windows 8 installations when I was evaluating it, and I distinctly recall the option to turn on the same tracking features that 10 had on by default. In the Windows 8 installer they were presented up front and off by default, in 10 they are hidden and on by default.
As of a month ago this is still the case; I reset a Windows 8 hybrid laptop, going through the standard installation screens, and then upgraded it to 10.
But, as you noted, a lot of the telemetry instrumentation has been backported so that argument is lost for someone wanting to avoid upgrading to Windows 10 from an earlier version.