I think you are vastly overvaluing how much consumers actually value phone thinness. The majority of consumers use phone cases (most modern phones have a camera popup specifically to be better compatible with a case to this end) so I think what customers value the most is lighter weight - not smaller form factor. A replaceable battery does come with a slight compromise to weight but stopping the endless chase of thinness has several engineering advantages when it comes to ports and cooling.

I don't think your speculation is completely unreasonable, but I just want to point out that consumer preference as revealed by current, actual reality only provides evidence in favor of my side of the argument. It's totally possible that the manufacturers are completely wrong about consumer preference and they are acting against their own interests by making the batteries non-replaceable, and somehow none of the manufacturers noticed this or were able to successfully take advantage of it to gain market share. But, I think that would be a pretty surprising thing if it turned out to be true.

Usually, in consumer electronics, the unencumbered market tends to gravitate toward what people actually want to buy. Totally possible this could be an exception to the rule, but I doubt it.

I would prefer a phone that was robust enough to not need a cover, because covers add a great deal of size and weight.

In the absence of such phones, I compromise on adding a cover.