Plus, isn't that the whole point of the mRNA vaccine as opposed to a dead virus vaccine? It is changing the cell composition through the RNA so that the protein spike is embedded in the cell memory and through cell reproduction. So it shouldn't wane because the memory should be reproducing. Unless, of course, it is just a placebo.
No, this is not how mRNA vaccines work. They work as follows - mRNA is injected into the muscle, and then the mRNA goes into muscle cells. mRNA is translated into protein (spike protein, in this case), which then goes to the surface of the cell. The immune system then reacts to the spike protein. The mRNA then breaks down and is eliminated. So it's just a different way to present the spike protein to the immune system. It has nothing to do with "cell memory" or longevity of response.