It's not so much that vaccination is iatrogenic, it's that when you have a sizable percent vaccinated, but a sizable percent not, you're kind of in this worst of all possible worlds from an evolutionary standpoint.
If worldwide vaccination rates were very high, it would drive viral transmission down to negligible levels and effectively slow the rate of evolution by decreasing the viral population size to something very small.
If there were no vaccine, we'd be back where we were, but there would be no vaccine to evolve against as a selection pressure.
When you have, say, 50% vaccination, you have a sizable population to evolve in, and the vaccine as a pressure selecting random mutations.
If the vaccine is very effective in preventing asymptomatic transmission as well as symptomatic infection, it might not be such an issue. This might be why the delta variant seems to not be doing much in vaccinated individuals. But it seems vaccinated individuals do still transmit the virus asymptomatically at some nonnegligble rate, and then you have a sizable nonvaccinated population.
What I'm worried about now isn't so much the current state of the delta variant, it's where things are headed. Specifically, how this evolves in (1) the unvaccinated populations, and (2) what the future might hold for vaccine resistance in the future.
It's one reason I'm sort of angry with the FDA for playing this power game about approving a booster vaccine. It's not their job to evaluate the need for the booster, it's their job to evaluate the safety and efficacy of it. By the time they change their minds about need, it might be too late.