in my experience, it's the charge port that goes. My last two devices, each of which I had for 3 years, the charge port failed before the battery did. I just finished fixing my current one. Although I did decide to do a battery replacement anyways while I had the device open. But that was just because of how hard it is to open them at all. My battery health monitoring app said it was at ~80% health still. And while the charge port hasn't always failed on me in the past, I'm not sure the battery has ever been the reason for me personally, although I'm probably just lucky in that regard.

Basically, I'm not sure that any one "thing" is the thing that always fails. There are multiple common points of failure and generally speaking, phones are hard to service in any manner these days. I personally would gladly give up water and dust protection, as well as thinness in exchange for ease of serviceability. Unfortunately, that seems to be a relatively uncommon set of preferences (although obviously it's probably much more common in places like Hacker News).