Why do you feel you have to replace the phone if nothing is wrong with it?
"I just can't imagine having to replace the entire phone instead just because a $20 part wore out, as batteries do."
I consider a battery that has reached the end of its useful life to be something wrong with the phone. If I cannot get at least a decent day out of the phone, it's not of any practical use to me any more.
When the battery wears out on my Note 4, as it has already done, I drop $20 and get that new-battery battery life. Rip off back, pull old battery, pop in new battery, start up phone. Done!
When the battery wears out on a [insert model of sealed-battery phone here], I have three choices:
1. Replace the phone for many hundreds of dollars.
2. Add bulk to my phone by adding an external battery case.
3. Hope I've picked something with a relatively good iFixIt score, buy a battery and specialty tools, set aside an afternoon, and hope I don't mess up my phone digging the old battery out.
NOTE: I consider a factory-sealed phone with a really good iFixIt score to have a replaceable battery. I just swapped the battery on my Nexus 7 2012 and it was almost as easy as swapping the battery in my Note 4. Took an extra half hour and required a few specialty tools, but I was able to replace it with almost no risk and very little fuss and bother.
The Note 5 had a terrible iFixIt score. Mostly because they glued the back on and it takes a heat gun to soften the adhesive. Once you have done that, it's actually not a bad battery replacement. But, heat gun on my phone = dealbreaker.
If they make the Note 7 so the back comes off reasonably easily without applying a lot of heat, I'll reconsider.