Our body has been using mRNA for at least hundreds of millions of years if not billions. (I'm not going to bother to look up how far back it goes.) We have DNA to mRNA proteins, we do not have mRNA to DNA proteins.
Why should our body suddenly start doing something that has never happened? The mRNA vaccines are simply slipping some bogus production orders into the job queue, they go nowhere near the master blueprints. There are various forces that would like you to believe otherwise. Mostly originating in Beijing and Moscow.
You’re aware that mRNA is a fundamental part of DNA based life biology, so there are lots of long term issues, by design of biology. Just probably not the ones you’re thinking of.
I would note there’s billions upon billions upon billions invested in developing cancer treatments across hundreds if not thousands of research labs, not just Moderna. This isn’t an all eggs in one basket thing.