Not trying to be Mr. Unpopular here. I just wanna share a perspective from a manufacturer's standpoint. MI manufacturer’s (at least internally) consider themselves in the consumer product market. At least the bean counters do.
Obviously a manufacturer could do anything they want but everything has a cost. Cost affects sales and sales affects keeping retail prices down. Most products get a price fixed to them when they are designed. Replaceable batteries have a cost which means if they add that as a feature then something else will get left out. So decisions get made.
Everything you say is true. But also consider what happens to those batteries. User-replaceable ones can get recycled, I wonder how many of the ones embedded in throw-away products are disposed of properly.
If Company A decides it can save a few bucks by cutting particular corners, that pretty much forces companies B and C to follow suit. I get that. The onus is not so much on companies because they give the customer what they want, but on customers who care only about the lowest price (and then complain when their plastic case breaks).
As to phone batteries outlasting phones, that may be true for most people, who can't resist marketing that's designed to make them feel inadequate if they don't have the latest and greatest.
. So yeah, I'm an outlier...my iPhone 7's battery went nuts, and after getting a new battery, I'm still using the iPhone 7 and it can run iOS 14. It's an effing phone, for chrissakes. I don't care if it takes 38 milliseconds longer to load a web page than an iPhone 11.