This doesn’t tell us much. Could the batteries have been larger and/or the device even thinner if the batteries weren’t user replaceable? (I don’t know the answer, but it seems plausible.)

My old phone was as thin as my new one but it had a larger batter and was replaceable. I always chosed the replaceable battery first if I had a choice. Last time however I didn't find a phone that had a replaceable battery. I usually use my phones a lot longer than most. My first one lasted for more than 10 years with several battery changes (Nokia 3210)