They teach the immune system how to fight the virus, reducing duration and quantity of infection, which reduces opportunities for the virus to replicate and potentially mutate.

A non-sterilizing vaccine only increases the evolutionary pressure on the virus to evolve to be vaccine-resistant. Such a vaccine more or less becomes like taking a half-course of antibiotics.

Hmm, is that really so? As far as I understand, for bacteria a more significant factor is horizontal gene exchange (because their genome mutation rate is much lower, presumably because they are more complicated than viruses).

Viruses can't do gene exchange, so the only factor for developing resistance is the total number of viral particles produced, so even if it's a weak vaccine or half dose of antiviral drug, it would be helpful

At a population level view he is exactly right.

A partial dose of antibiotics scenario is exactly what we have done with these vaccines.