By your reasoning the eradication of smallpox or polio (in developed countries) would have been impossible. The human immune system and environmental barriers

By your reasoning the eradication of smallpox or polio (in developed countries) would have been impossible. The human immune system and environmental barriers between hosts are the primary selective pressures on the virus. Those exist in the absence of any countermeasures. If it was vaccines driving selection then the more infectious strains would have emerged in areas of the world that had early access to vaccines. That is not what happened. Vaccines squeeze the reservoir of susceptible hosts where mutations can occur. Also my understanding is that the SARS-CoV-2 genome is relatively stable compared to other viruses as Coronavirus RDRP has pretty high fidelity. But I could be wrong about that last one.