Same here. Had no plans to update our phones, but my wife's phone 6S+ became unusable under 40% and my 6S kept dying around 20%.
Ended up getting iPhone 8's over Thanksgiving only to have it revealed new batteries would have solved our problems.
I'm in the same boat. Got tired of my 6s+ being slower and slower, so I upgraded to the 8+ when it came out, and subsequently to the X. But I honestly had expected to hold out another year for a new iPhone because I loved my 6s+ before software updates started slowing everything down.