I hate to quibble but "cosmic rays" are high-frequency radiation, in the X-ray band, gamma ray band, or even higher. Being photons, they are not affected by the earth's magnetic field. They are generally stopped by the "ozone layer" in the atmosphere, which is one good reason why we ought to worry about that layer disappearing.

The earth's magnetic field does indeed deflect charged particles (alpha particles, beta particles and other shredded atomic nuclei) emanating mainly from the sun. The loss of the earth's magnetic field will allow these to hit the earth's surface, which will not be good news for delicate electronic equipment.

(It also means that charged particles, which break up ozone, won't be directed towards the magnetic poles, so ozone layer loss will be less severe but more widespread, or so my reasoning goes).

The geological record does not appear to correlate mass extinctions or "species bloom" with magnetic pole reversals.