What's the point of all that effort if those habs are going to be one-way trips for the people going there? What are they going to accomplish in their floating or underground habitats? They're not getting back to anywhere useful from either place. They're not going to survive without frequent Earth resupply[1]. Whether habs could exist is an interesting thought experiment, but that's all it is for the foreseeable future.

Even a sizable Mars colony probably won't survive without frequent Earth assistance. The Mark Watney fantasy of growing food in Martian dirt with a little added fertilizer: mostly debunked[2]. The most sustainable case—for the foreseeable future—is probably a Biosphere2-like environment, where everyone hopes there's no accident, sabotage, or environment-caused damage. How many Starship missions would it take to get enough materials to Mars to build one Biosphere2 to support 8 people?

I agree with you that meaningful colonization of Mars is not serious. I just think the prospects on the inner inner planets are even more absurd than on Mars. In a floating Venusian hab, you could generate breathable air and not much else. Underground on Mercury, getting breathable air might be a problem, but you can import—at great cost, that delta-v is brutal—anything you can fit on a suitable rocket.

There's no point to any of this except as research stations or jump-off ports, and for that Mars is the obvious choice: we can make hydrocarbon fuel there, and the surface isn't equipment-melting. But where would we be jumping off to? We have nothing planned, and no particular reason, to send humans to Europa or anywhere else.

[1] How do you resupply a floating habitat on Venus? Even if you could, the constraints and resource limitations of a floating hab would be even more severe than for an underground, resource-poor Mercury hab.

[2] https://www.sciencenews.org/article/mars-farming-harder-mart...