“Musk believes we have about 15 such opportunities (roughly 30 years) to establish a self-sufficient civilization on Mars before Earth potentially faces civilization collapse.”
Mankind has no practical (or even impractical, frankly) way to shield astronauts from the kind and quantity of flesh- and brain-cooking cosmic rays they will inevitably absorb on their way to Mars, on Mars, or on their way back from Mars.
Why are we entertaining this fantasy?
Your point is valid with current technology.
We still need major advances in nanotechnology/materials technology to address these issues—hopefully maybe only ten years away...