You are correct, of course, about battery health decreasing over time. The problem wit respect to diagnostics and replacement is that with respect to my iPhone, the Apple store told me last year they decide to honor the reduced price battery replacement only if battery health is below a certain level (80% if I remember correctly) and that i should check battery health periodically before making the trip in for further diagnostics and replacement. So “battery health” made all the difference with respect to whether i paid $29.99 or $79.99 to replace my iPhone battery.
My understanding, based on what other people were told, was that they would replace the battery regardless if you had a 6 or 6S. And, they can also do diagnostics over the phone.
My observation here, since that feature was added, is that lots of people are obsessing over the battery health number. My advice to most people is ignore it unless 1) you're having issues or 2) you're within a month of so of the end of your warranty.