I've never had a problem with any device that has a sealed battery. My oldest phone that I own is a iPhone 3GS that still works and holds a charge as it I just bought it today.
I've never had a phone with a built in battery shut down at 15% like I did with phones with removable batteries.
I still have:
iPhone 3GS
iPhone 6+
iPhone 6s+
iPhone 6S
iPhone 5S
iPhone SE
Note 5
3 S7 Edge phones
All of them have amazing battery life. I think you are making a big deal out of nothing.
I'm in agreement with paradox. I have several Galaxy Tabs from 4-5 years ago, my wife has a 1.5 year old S6 and while they still work the battery doesn't last anywhere near as long as when we bought them. SOT is probably half of what it was.
Lithium ion batteries degrade with use, scientific fact. With my Note 4 when the battery doesn't hold a charge as long after about a year I pop in a new one and I'm back to fresh new battery life.
Way easier than trying to crack open a sealed phone to replace a battery. And definitely contributes more to device longevity.