I replaced my 6S's battery myself for $26 and 20 minutes of my time just before Apple announced the discounted replacement offer. I might still do it myself since it was more about the personal time savings than anything. Still, I think the only real issue is that Apple didn't disclose the severe throttling for the sake of stability (I estimated about a 30% drop in performance from my crude testing).
My old battery was tired, it had hundreds of cycles on it, but I was surprised to see Coconut Battery claiming it was still at 89% capacity. Even throttled, the old battery struggled to make it through a day. I don't blame Apple for the fact that the battery was shot--I just wish they had told us why our phones felt like they were running so slow. They let iOS11 take the heat, when it was the throttling, not the OS. Interestingly enough, I have noticed a few app crashes, though that just started happening now, almost 2 months after I replaced the battery.
One thing to consider is how much abuse our phones get. They ride around in a 98F pocket, are exposed to the elements and temperature extremes, get beat around, and probably see a least 4-5 hours of use every single day. After a couple years, that's easily 3,000+ hours--probably way more than that for many people. We probably don't ask that much out of any other battery-powered device we own.