The alternative explanation does not change P(severe infection|vaccinated), so doesn't change the nature of the choice to get vaccinated. But it would imply, if

The alternative explanation does not change P(severe infection|vaccinated), so doesn't change the nature of the choice to get vaccinated. But it would imply, if proven, that the claim that vaccines reduce severity is false. If the alternative explanation is proven, it implies that people who are vaccinated and develop an infection should react to that news the same as an unvaccinated person. In other words, even if the alternative explanation doesn't influence the choice to get vaccinated, if it is true and we are saying it's false it would mean vaccinated people are more likely to die once infected because they've been led to believe it is not as serious a concern to be infected and will be less likely to, for example, notify their doctor as promptly or go to the ER given they have been informed their risk of severe infection is low.