It's not as convenient as a user replaceable battery, but most phones have batteries that are replaceable by repair shows cheaply.
Weirdly my 3.5 year old phone actually has about the same battery life as it started with. As android 3.5 years ago was terrible at battery management, so software updates have compensated for battery degradation.
But all those replacements seem to be with someone pulling apart the phone's components secured with glue. It looks extremely risky and likely to cause damage to phone or components like display ended up getting scratched.
Also, are original spec batteries available easily? I don't want to put something which is not the same as it might cause some battery related accident.