Not just the fishing.
IIRC the Cuyahoga River emptying into Lake Erie caught fire once upon a time.
That's true (it caught fire a few times), though they have cleaned it up.
The Great Lakes have already been screwed up.
Historically the only game fish in the lakes should be Walleye, Sauger, Perch Walleye, Muskie, N Pike, Whitefish, Lake Trout, Brook Trout (Char), Smallmouth Bass and Artic Grayling.
The Salmon/Steelhead Rainbow were added to because of the death and decline of many of the species listed above.
Yet it is still a thriving fishery. If the carp take over, it won't be.
I don't see Asian carp being useful for anything other than fertilizer. In addition, there are
a lot of rivers which are tributaries of the lakes and many of them are threatened as well, along with the smaller lakes which are a part of the watershed.
So far, the only Asian carp which have been found are grass carp (which, as the name implies, eat vegetation, and were brought in by idiots to reduce vegetation in their freaking ponds), which aren't themselves as dangerous to the indigenous species. The plankton eaters are the ones which will really damage the fishery.
I don't think it wise to just throw our hands up in the air and give up.