I think you are just deluding yourself once again by ignoring the BALANCE of trade and focusing exclusively on selling wheat.
<<If Euros are the most popular , we will get some , the same way we got Pounds and Spanish dollars when those were the big currencys.>>
You got them by buying them. You still get them by buying them. What you don't seem to be able to grasp is that you will always be able to buy them, but they will cost you a lot more when the U.S. dollar is no longer the international reserve currency. You focus insanely on how much wheat you can export. And you ignore completely how much other stuff you have to import, from oil, dry goods, automobiles, metals and minerals.
If you want to compare the U.S. economy of the 18th century with the 21st century, the first thing you would probably notice was that the U.S. was exporting more than it imported. Today the reverse is true. So whatever you export in grain, every dollar earned will go back out to pay for imports, and then there still won't be enough money from grain and all your other exports to pay for all the imports. So what happens? You need to sell dollars. But if the demand for dollars is low, you have to sell more dollars than you sell now to get those euros. The more expensive the euros are that the importers have to buy so they can pay for their imports, the more money they have to charge for those imports. Invariably your standard of living will go down. Because whatever goods consumers are enjoying now, less will be able to enjoy next year because they won't be able to afford them at their new prices.
You say your economy did fine back in the day. Sure but you're not back in the day. Your population expects more in the way of worldly goods than they did back in the day. What's gonna happen is their standard of living will shrink. If you're nostalgic for the good old days when Americans lived simple lives down on the farm with little luxury in their lives, get ready for a return to those days. But don't think the population is going to relish the change. Gasoline unaffordable. Holidays and trips unaffordable. Foreign clothing? unaffordable. American clothing? pay more, get less. Make do with less. Medical care? costs more. It's like a thousand little cuts. It adds up to longer working hours, less to show for it.