I would be surprised if viruses using U instead of T didn't already exist. After all, don't all viruses work by doing gene editing in vivo, except just localized to one cell?
EDIT: well, I suppose the question is whether cells of living beings could produce the U required for the viruses. But if not, then a wild virus using U instead of T to bypass our immunity also would not be a threat for that very reason.
It’s not the use of Uracil/Urimidine that bypasses the immune system. RNA uses Uracil instead of thymine in all organisms afaik, and RNA viruses certainly exist. It’s pseudouridine that’s the magic stuff.
EDIT: well, I suppose the question is whether cells of living beings could produce the U required for the viruses. But if not, then a wild virus using U instead of T to bypass our immunity also would not be a threat for that very reason.