Dead iPhone. Battery was tested near optimal days earlier

So I took my iphone to Apple. I complained that it was getting way, way too hot while charging. It often wouldn't detect the original OEM cable. The battery would be exhausted over night. It often wouldn't connect to iTunes either. All this started within a few days of charging my phone at a Best Buy charging kiosk.

The genius said the phone is in fantastic shape and he was surprised how much life I have in my battery: 95%. He asked if I had my battery recently replaced, I said no. However, my device was replaced within weeks of my Apple Care ending, so over a year and half a go. Apple's advice was to bring it back right away if you have the same issues. This time though, he said do NOT do a DFU restore, Doing so removes all troubleshooting codes.

I bring back my iphone after it was completely dead. The rep took it in the back and tried different things. He said that the phone is unfortunately dead. Based on the last tech's report, the battery is fine and he told me not to bother with a replacement battery. I don't know if he took the old battery out and inserted a new just for the sake of it.

My question now is what would be causing this failure? Is there a fix or is everything so integrated that you can only replace the mobo which wouldn't be worth it?