Lots of Apple devices suffer from the innovation of Apple Computer, including the iphone/ipod touch and macbooks.
First, let's just say that non-replacable batteries certainly are replaceable. I've had the batteries replaced (with new ones) in my 2011 Macbook Air, and two 2012 iphones. This happened in 2016.
Batteries and heat
See also the idiotic approach to OSX cooling
Coconut Battery App for OSX and IOS
Coconut Battery is an OSX app which provides useful battery information in a GUI, and can also display battery information about connected IOS devices.