Just my opinion for what it's worth: the only good reason I would have for not upgrading to Windows 10 is if the device is old and drivers are not available, which seems to be the problem with some old laptops in particular, or there is insufficient memory or drive space.
If you spend some time after upgrading, disabling most of the extra 'features', Windows 10 is hardly any different from 7. There is plenty of help available online for doing this and Win10 seems far more easily customisable than its predecessors. Most of the so-called features are trying to compete with Android and regain the ground Microsoft lost to Google in the mobile market.
Any modern operating system will try to harvest data from you, but you can minimise this by altering various settings. Admittedly you may find settings revert to the defaults after a major update but I've ony found that once since I upgraded last year.
Mike.