Weirs are different than ladders. As Neebo said, ladders are a way for fish to make it around a dam so they can continue moving upstream i.e. salmon and steelhead that move out of the lakes and into the rivers to spawn. Weirs, on the other hand are barriers designed to stop movement upstream. Weirs are, I believe, specifically designed to take advantage of some aspect of the thing they are trying to stop. Lamprey weirs are the ones I am most familiar with.
Initially, they tried to use electrical barriers, but it interfered with trout and salmon and was too costly to operate. After some research, it was determined that a couple of things could be done to stop lampreys but not interfere with salmon and steelhead. So, they introduced barrier dams with a metal lip on the top - lampreys can't jump over them, but salmon and trout can. Also, pool type fish ladders were designed that created currents that were too strong for lampreys to swim against but still allowed salmon and trout to get through.
Also, by putting the lamprey barriers in place, it greatly reduced the length of rivers that needed to be chemically treated to control lamprey spawning, making the chemical control process more cost-effective.
My take -
1) These fish will get here. Just like lampreys, zebra mussels, gobies, etc.
2) No single method will either prevent that from happening nor control it once it does
3) There will most certainly be negative consequences when they get in the Great Lakes, and there will also likely be unforeseen positive consequences. Until those consequences materialize, it adds nothing to the conversation to dramatize the possible effects with hyperbole. We can guess all we want, but we cannot know. Hyperbole delegitimizes concern as a good portion of the folks receiving the hyperbole will immediately discredit it and move to the exact opposite pole, thereby producing a result that is the polar opposite of what was intended. We can and should be concerned, but, as they say, "reports about my death are greatly exaggerated."
What we should do -
1) Continue to raise awareness, not by over-reaching, but by raising the issue and the uncertainty around the consequences in a realistic manner that is fact-based as opposed to specualtive
2) Continue to press for education about invasive species in general
3) Press for greater enforcement of measures already in place to reduce the threat of introducing invasive species
4) Continue to make a concerted effort to not only study what will likely happen but also how to MITIGATE it once it does, as opposed to focusing on "all or nothing" and "the sky is falling we are all doomed" positions
Just my thoughts.