Batteries aren’t a good reason to upgrade imo when they’re replaced with new for $70 or less. And I haven’t heard of thermally damaged components in phones. Is that a thing?
There is almost noting in the way of cooling measures inside smart phones, and I've consistently had multiple iPhones become hot enough to the touch that I removed them from my pocket and set them on an open surface to chill for a while.
Oftentimes, the phone doesn't even seem to be doing anything visibly taxing. Probably just indexing and the like. But with zero cooling fans, heatsinks, or even air intakes, it isn't so out of line to suggest that heat damage over time is going to render the components less reliable.
My 6S overheated once while charging, and later that day, while shooting a video, it glitched while finishing writing the data for the video file, which in turn the corrupted the entire camera roll. It would attempt to restore, and get all the way to the last video, fail to be able to write it, and then become corrupted all over again. The only solution was to wait several hours for it to attempt to restore again at an Apple Store, and then when the video popped up in the camera roll, they immediately deleted it before it finished attempting to save.