I think the latter would make more sense. The tray-less phone has a plastic ”filler” part to populate the empty space in order to retain structural robustness. In manufacturing alone it is always more cost effective to have a single variant.That’s already happening and several people have complained about it in this forum.
The other part of the explanation that I forgot to mention is that Apple seems to believe that international travelers are a small minority of the US user base not worth caring about. Thus, under that assumption the vast majority of the US users would be unaffected by the lack of a SIM tray. And it seems to be a safe assumption: indeed the vast majority of americans don’t travel outside their country. Selling the iphones without a SIM tray would harm a lot more users, and hurt sales much more, in other regions of the world than it does in the US.
Also, there may have been some kind of agreement between Apple and the US Carriers because it actually benefits them: if international travelers can no longer buy a local prepaid SIM wherever they go, at least for now they’ll have no choice but to use roaming.