The magnetosphere protects the lower levels of the atmosphere from charged particles from the sun and further sources. This isn't just solar flares, but a constant bombardment of energy. Without it the rest of the atmosphere will began to degrade, with specific worry about our ozone layer. Some animals such as birds are shown to relay on the magnetosphere to help navigate. Aside from the changes to ecosystems and climate, the magnetosphere is also responsible for the aurora borealis, so we would miss out on that.
On the plus side, we'd finally have enough data to answer questions about what sort of radiation shielding generational spaceships would need, and exactly how much cancer prevention Earth's atmosphere is providing.
https://en.wikipedia.org/wiki/Laschamp_event
> This loss of the geomagnetic shield is claimed to have contributed to the extinction of Australian megafauna, the extinction of the Neanderthals and the appearance of cave art. However, the lack of corroborating evidence of a causal link between the Laschamp event and population bottlenecks of many megafauna species, and the relatively moderate radio-isotopic changes during the event, have cast significant doubt on the real impact of the Laschamp event on global environmental changes.
Caveat: Even with the atmosphere fully present, the magnetosphere does indeed stop many charged particles that a gas barrier does not, and this would definitely be an immediate problem to surface life to some extent (how much is debatable however). We'd also be much more susceptible to electronic and electrical grid damage caused by a much larger percentage of solar storms that would have previously been too weak to do much because of our giant magnetic shield..
It's not an humanity destroying event, but it is a big stressor on a world already going through a lot.
"Other sources estimate that the time that it takes for a reversal to complete is on average around 7,000 years for the four most recent reversals."
A range of 2k-12k is also in the introduction.