I am a big fan of YIMBY-ism but I think your points here are fair. This is a controversial take among YIMBYs but I think the benefits of ADUs are oversold. Of course I am all for allowing them if that is all we can do but the idea of the ADU only exists because it is supposed to allow building more housing while preserving the overall "character" of single family neighborhoods. Subdividing a lot and building a new home is better in every way for both the homeowner and the resident of the eventual structure. ADUs seem like they could provide a lot of housing because it is a huge increase in what is theoretically allowed but it is not a huge increase in what people will realistically build in the United States in 2025 in single family neighborhoods. This is true for the reasons you state and also many other reasons because it is the opposite of economies of scale that typically brings costs down.

We have an answer to building more housing at a lower cost and it's apartment buildings. We need more apartment buildings. We don't need them everywhere, there will still be plenty of places with predominantly single family homes, but we need them in more places than they are today. They don't have to be big high rises but there needs to be a enough number of units in a single building to make projects pencil.