Barring a time machine, the "is it safe ten years from now" cannot be answered conclusively today. We're left to use other available evidence to make a risk analysis, like "mucking with the immune system tends to have effects that show up fast" and "what do we know about mRNA in the body, especially when delivered by viruses?". There's a risk to inaction, too.
We don't look at evolution and go "welp, can't test evolving humans from microbes, so we must avoid drawing any conclusions".
Barring a time machine, the "is it safe ten years from now" cannot be answered conclusively today. We're left to use other available evidence to make a risk analysis, like "mucking with the immune system tends to have effects that show up fast" and "what do we know about mRNA in the body, especially when delivered by viruses?". There's a risk to inaction, too.
We don't look at evolution and go "welp, can't test evolving humans from microbes, so we must avoid drawing any conclusions".