Once again, this confounds individual case management with public health.
In terms of a given patient, the effectiveness and mechanism of action of mutagenic pharmacology still seems up for debate. But even assuming that such drugs are appropriate treatments for sick individuals, these are administered in cases where patients are diagnosed, ill, isolated, probably hospitalized, and not circulating in the general population. The objective is to lower the viral load in their own, individual bodies, hopefully so they recover. There is no goal of purposefully circulating further infections with some sort of hobbled virus.
Even were that to inadvertently happen, the epidemic would continue to spread unfettered from the fitter at large clads of the virus, which would crowd out any less transmittable variants. This is what happens in any event; there are always dead end variants which come and go.
Giving everybody a weakened, ineffective form of a virus, is a form of classic vaccination. The reason people have to be poked is because community spread will not happen by contagion. Far from cridible, that Sanford suggests circulating mutated viruses as a public health policy is an embarrassment and speaks to the shallowness of his understanding of epidemiology.