The waterproofing in particular makes me think this is likely. Many average consumers care way more about having a device that will survive being dropped in the sink now than they do about replacing an 80% capacity battery in two years.
But I also question your premise, I don't think it's as clear. I've never dropped my phone in a sink, but had battery issues with basically every phone. Often the battery starts failing much earlier than 80% in two years.
To me it looks like water proofing is being used as an excuse to add built-in obsolescence to products, since I imagine that conformal coating would do the job better. What am I missing?
They did just fine with replaceable batteries.
The main faucet to your house sits under 60psi water pressure all day, and the moving parts are removable. This is 100 year old technology.
The falloff has to be way more than 20% for two years?
Keeping it within 20-80% most of the time might keep the battery healthier.