I can give that a stab, if you like.
I don't think it's inevitable, but I don't think it's unlikely either. It's a real risk. Even if we went for a physical barrier, which many people are seriously advocating, as I understand it, it might be a decade or two before it's actually accomplished. The engineering isn't as simple as a few dump truck loads of fill. There will be lots of things that need to be done. We need a plan, at least over that time scale, even if there's eventually going to be a physical barrier.
There are also other aquatic connections besides that canal. It's been discovered that in the headwaters of various rivers in springtime, when water is high, and even possibly in marshy areas in the headwaters, carp might actually spawn there and the larvae go in both directions. There are other connections that the U.S. Army Corp of Engineers are trying to discover to actually make physical alterations to prevent—