We have 45% of Americans refusing to get a vaccine now, imagine the push back if we are attempting to get people to take a new one every 6 months. I don't have a better answer I just know that absolute chaos is down that road.
Also, some people are not antivax, they are afraid of new mRNA technology. When the technology has had more time to bake, they will feel more comfortable.
Finally, Novavax will be coming online soon. That should help with some of the mRNA skeptics.
then they can get the Janssen (aka Johnson & Johnson) vaccine which is not based on mRNA but an Adenovirus vector.
Viral vector has only been used before for Ebola. All US vaccines execute custom RNA code that was spliced from the real virus. Viral vector just gets that code in the cell by a different method.