Main problem with updating old PC to win 10 is driver support. Hardware makers don't support their stuff forever so there is a chance you won't have drivers for some stuff.
I don't see why you can't stay on win 7. All you lose is support and no one should be asking Microsoft for help as half the time they don't know. No more patches means no more chance to break windows.
Even though MS said anyone should use win 10, they aren't the ones who have to deal with the broken pc. I myself wouldn't upgrade to win 10 on that PC if it works fine now.