The commerce through the open seas being so cheap is, sadly, a fiat phenomenon. It was never that cheap to trade over such large distances. It's largely a function of Pax Americana (and Pax Britannia before it) and it's heavily subsidized through the dollar hegemony. It will be more expensive when that hegemony collapses, and we can kind of see it in real time with the Strait of Hormuz. That said, I believe there are much cheaper ways to have free trade through the seas, and instead of relying on the US Navy, entrepreneurs will figure out something much more effective (like Nike shipping containers only of left shoes or right).