I don't think anyone can deny the benefits of having USD as a default currency for global trade as of today but just think once from the perspective of a country against whom USD is used as a political/economic weapon. Iran/Russia won't care much about the points you mentioned. They would come up with some workaround/compromise instead just to keep things going.
USA has a great currency but they use it as a tool of bullying others way too often. Other countries have started noticing and are now taking concrete steps to come up with an alternate/backup plan.
And it may not have a direct effect on US economy if some other currency becomes dominant but they would certainly lose out on their political dominance (they would no longer be able to strongly impose economic sanctions and coarse others to do as they please) and that could have snowball effect on its economy.