I think there are some false equivalencies in here. There are real serious challenges to be overcome, especially (as you say) dealing with the radiation exposur

I think there are some false equivalencies in here. There are real serious challenges to be overcome, especially (as you say) dealing with the radiation exposure over >= 3 months in deep space, and we've seen the serious health issues Apollo astronauts have suffered from after enduring the same sort of exposure for a comparatively short time. But overall, the science and engineering are largely in place; to do this it will take scaling, integration, and refinement all on a massive scale, but it really isn't like e.g. going from the Wright flyer to SR-71. The latter was unimaginable back in the day of the former, which simply isn't the case for SpaceX's plan or it would be getting a lot more flak from real experts than it apparently has been (though, let's be honest, SpaceX are the real experts now). And, again, nobody's saying it's anything other than a long shot, not even Musk himself. But a lot of people seem to be pooh-poohing it for bad reasons. For instance, the awful quality of the audience at the Q&A does not in reality tell us anything about SpaceX's ability to execute on some or all of this plan. In fact, I think Musk was ready for much more challenging questions than he ended up receiving (how could he not have been?)