Not impossible, just not as probable as among the unvaccinated.

For evolution/natural selection to accidentally find a mutation that would make a more deadly virus variant there needs to be a huge population that is rapidly reproducing. Such mutations are actually rare.

The vaccinated are not as fertile ground for the virus to replicate in. The overwhelming majority of the relatively small number of breakthrough cases have been asymptomatic or mild symptoms, which means less virus replicating within them because their immune systems are more quickly limiting and eliminating the virus population.

The unvaccinated are the main pool of virus and are where large numbers of virus are rapidly replicating. Math says they will be the most likely (by some order of magnitude that I’m unqualified to compute ) more likely to breed a new, more lethal variant.

That doesn’t mean I agreed with relaxing the mask mandates, social distancing, etc. The CDC and the powers that be knew the delta variant was spreading rapidly and would inevitably get to the US. They shoulda known better.