For desirability - as somewhere attractive to live as well as ultimate defense against human extinction - I personally think spinning space habitats would beat Mars colonies. Mind, I doubt they are achievable either.
Asteroid/comet resources probably do include every known element but I would be very surprised if we will find all the ones a habitat needs as minerals we can extract easily; nickel-iron in great abundance, that can be a structural material with minimal processing but things like radioactive fissionable ones, for industry and medicine as well as energy seem unlikely except at trace concentrations. More likely to find concentrations of those on a planet that has a complex geologically active past, preferably involving water too, but we don't have evidence of uranium, thorium or other potential nuclear fuels as viable ore deposits on Mars either. I suspect a lot of critical elements will be at very low concentrations, well mixed. We will find out a lot more about what is actually there but we don't yet have a good inventory of mineral deposits - of what is present, where and in what forms. That alone tells me any commitment to colonising Mars is way premature.
From another angle I think
the productivity of human labour in space or on Mars looks likely to be significantly lower than equivalent on Earth. A Mars economy needs to be a lot more productive per person than equivalent on Earth because services we count for free or very low cost - air, water, radiation protection, productive soils - are in addition.
Apart from simple lifting in low gravity I expect every kind of physical activity will be harder, not easier - and if digging a hole with hand tools requires a million dollars
in PPE that seriously limits mobility and fine hand control a lot of simple jobs become slow and very expensive. Replacing the humans with machines adds layers of industrial complexity and seem likely to add to the costs and difficulties rather than reduce them.
Yes it would be incredibly expensive and I quite agree, nobody has come up with a way of making it pay its way, or at least bringing the costs down to something that humanity will be able to afford. The idea of mining other celestial bodies for minerals to use back on Earth never seems to me to stand up, given the enormous costs of the changes in momentum involved in interplanetary space travel. And I struggle to see what other economic opportunities there would be. Energy is an obvious one, I suppose, but all you need for that is satellites to capture sunlight.
For all that it is currently out of reach I do think Asteroid mining - nickel-iron is the best we are likely to get and by any ordinary standards a remarkable resource - and I think some of the problems of exploiting could be solvable.
"Changes in momentum" is a good way to put it - and good to talk to people who understand that even in-space transport is a high energy activity that will always involve far more fuel/reaction mass than payloads. Yes, push on something in space and there is no friction and it keeps on moving but it won't be going where you want; to get it to a specific destination takes significant energy and reaction mass - and complex technology.
Achieving ultra-reliable in-space rockets that, once launched, don't use anything consumable that isn't produced on site as part of the mining operation would be crucial. Solar electric Arcjet or similar, using water as the reaction mass maybe. I would look for carbonaceous chondrites rather than M type metallic - they can contain significant quantities of nickel-iron as grains and nodules within a softer carbonaceous matrix, which in turn contains significant amounts of water. Which by quantity would be the principle thing being mined and refined.
Of course any asteroid mining operation would need to be ruthless in eliminating any need for astronauts - nothing adds to the complexities and costs of doing things in space than having astronauts. It would mean asteroid mining would not, in and of itself, advance goals of human habitation of space.
Discussing asteroid mining here might be a change of topic - not sure yet how tolerant moderators and participants here are.
