Keep them on iOS10 for as long as you can. Consider doing the tvOS profile trick to prevent the update from auto-downloading.i posted my screed before apple announced that they would replace batteries that still pass their diagnostics. my main gripe was having to go through the hassle of convincing apple that the battery needed to be replaced.
in light of the new policy, obviously i think it's better to pay $29 to apple to just do it, assuming they really do replace the battery no questions asked.
the elephant in the room is that i believe they are still needlessly crippling older phones. as i mentioned my kids iphone6 phones are still fine, never unexpectedly shut down, run fast... and are running ios10.
My 6 (with a lousy battery at the time) went from extremely usable to garbage as soon as I updated from 10 to 11. And since they stopped signing 10, there's no going back after you've updated.
The new battery brings my 6 back to where it should be, speedwise. Which is slightly slower than how it performed under 10.