Hmmmm. That must be why the Poe Lock is 1200 feet long. And why I've personally seen ships under flags from Europe and Southeast Asia travel through that lock.
It is 1200 feet long to accommodate all vessels in excess of 740' in length, which is the maximum size the smaller active MacArthur Lock at the Soo can accommodate. The Salties going to Lake Superior can use either lock. I think there's a width restriction on the Mac Lock, though.

The US Army Corps of Engineers has had plans on the shelf for years to demolish the two smaller locks and construct a lock even larger than the Poe, to accommodate newer, larger ocean-going vessels.
This is incorrect. The USACE is demolishing the older and smaller Davis and Sabin locks so it can build a new 1,200 x 110 foot lock identical to the Poe in order to accommodate US and other large Great Lakes vessels, should the Poe have a mechanical problem. Otherwise all boats over 740' would be stuck either in or out of Lake Superior. In addition to the 13 1000 footers, at least a dozen other vessels cannot use the Mac Lock due to their length - River Rouge traders James L. Oberstar [[formerly the Charles M. Beeghly), the Lee A. Tregurtha, and the Kaye E. Barker come to mind as a few of them. Canadian and international vessels would get the benefit of the use of the new lock, but it is NOT being built specifically for them.

If your theory were correct, you might want to explain zebra mussels and sea lampreys, which are not native to the Great Lakes, but most definitely native to saltwater environments.
Until you explain your theory of zebra muscles or sea lampreys being in excess of 740'x78', then my theory of them arriving in international vessels at or below that size will stand.