I'll admit to not understanding why, but "consensus" seems to be that the vaccine confers greater resistance to variants than does natural immunity- perhaps the natural immunity is over-fit to the one virus, whereas the spike is a necessary component to the whole family?

That said, I've also seen oft repeated that what you get is immunity*, aka resistance to the worst symptoms. Mild infections are still to be expected as possible, and Covid 19 will join the ranks of common-cold causing organisms.