J.G. is a famous case and his death definitely delayed development for a decade or more, but it should be noted that when he died, the technology was extremely untested in humans.

We now have a few hundred million people who got mRNA vaccine and had no trouble. I don't think that even after a death, mRNA would be completely toast. It has already proven its merit.

mRNA?

J.G.'s death involved an experimental modified adenovirus (the same family of viruses used as the vector for several non-mRNA COVID vaccines: Johnson & Johnson/Janssen, Astrazenica/Oxford, and Sputnik V).

The OP was mentioning J.G.'s case as an example of what could happen, too with rushed mRNA vaccines, and that the fallout would be bad.