Exactly this. Insofar as the vaccines actually worked to protect people who had never acquired SARS-2, an even somewhat vaccine-evading mutation has a sizeable population of real-SARS-2-naive individuals to spread through (i.e. the vaccinated who never had natural infection).

This is why I and others have opposed the concept of mass vaccination for an endemic highly-spreading respiratory virus. It simply doesn't make sense unless you can develop a vaccine comparable to natural immunity (both in magnitude of immune response but also robustness/resiliency to mutations; over the long-term the current vaccines offer neither).

SARS-2's risk is incredibly well bounded; we have a great idea of who is truly vulnerable and who would have to get struck by lightning 8 times in a row to be seriously harmed. We should have recognized that fact from the beginning; but recognizing that fact runs counter to the narrative required to scare the whole population enough to accept lockdowns/etc.