The immune system does this all the time. They're called natural killer cells (NK Cells) and they regularly target cells that have become cancerous or infected.

Well yes, NK cells do that when we're infected with a malignant infection.

For the last hundred years, or so, vaccines have been non-malignant, so we've never had to question such things.

However, mRNA vaccines are an entirely novel class of technology, so it's appropriate for us question:

a) how long mRNA vaccines persist in the body? b) how discriminating are mRNA vaccines in which cells they impact? c) if any testing was done on the side effects of accidental intravenuous injection of mRNA vaccines? (given that there are plenty of blood vessels in muscle tissue) d) if any longitudinal studies have been done on mRNA vaccination of humans? e) if any comprehensive studies have been done on pregnant or nursing women injected with mRNA vaccines, and the impact on their infants?

Of course, but so are almost all cancer treatments, and many cancers are still a death sentence.

I've said above, if there aren't other treatment options available, it makes sense, but even then I don't know why one wouldn't use standard immunotherapies instead, which are less risky.

I think Moderna and Pfizer generated a lot of profit from the vaccines, and now they need a way of turning that over into capital, and they are still trying to find ways to have widespread adoption of regular mRNA treatments. But even millions or billions of dollars of capital investment will not yield a profit if its a bad investment.

Even if that statement were correct what difference does it make if you're going to be in the ground in six months?

The difference between those 6 months being a slow grind that gives you the chance to say goodbye to the world vs. sudden onset death from overwhelming immune malfunction. Quality of life is not an afterthought, even with the end in sight.