Absence of a magnetosphere entails gradual erosion of atmosphere. You can still have one provided you're topping it up intermittently. So absence of a magnetosphere is not much of a challenge if you have a liveable atmosphere on Mars. But that's the actual challenge, right? Barring ancient reactors buried at the poles, it would take an unfathomable, multi-generational effort to produce a liveable atmosphere on Mars, be it by speed-braking comets in the upper atmosphere or nuking the polar ice.

The first step, presumably, is to set up a base on Mars and find a way to make enough money out of it (some kind of Mars-tech or billionaire eco-tourism or the ultimate season of Alone) that it could continue to persist. The first half of that sentence seems doable. The second part seems harder to sell (and is the plot of Thin Air). I think that's the vision Musk has. Or maybe had.

This isn't even touching on the legalities and ethics of interfering with the unseen biosphere that may already be there.

The colonise Mars thing has certainly pushed the Overton window to set the stage for other stuff much closer to Earth. A space hotel / research lab catering to the superrich seems pretty feasible in the next couple of decades. A permanent (at least for a decade or two) presence on the moon now seems like a strategic priority for both China and the US. Neither of those really seemed plausible 20 years ago. I think SpaceX changed a lot of that.

Anyway, I know you already know all of that. It's a pity to see what's happening to Musk.