I do not see how a geomagnetic reversal would be catastrophic, the suns magnetic field reverses polarity every 11 years.
IIRC it flipped sometime last year, and most people were oblivious to it.
If Earths magnetic field weakens it'll still protect us, if it completely vanished the Ozone will provide moderate protection(some people will get cancer). It would most likely take thousands of years of constant bombardment from the sun for the Earth to lose its atmosphere with no magnetic field at all. Venus has little to no magnetic field and still maintains its atmosphere.