(source data from the USDA's Economic Research Service)
Notice that others such as India are stepping into the gap. In 2008 and 2009, India produced just about $150 million worth of shrimp, but in 2013 they broke the $1 billion barrier, increasing exports to the US by a factor of six over just five years. It's definitely possible to have a cleaner production system: according to the Monterey Bay Aquarium's "Seafood Watch" guide, the best choices for shrimp are from aquaculture done in North America. It is much less damaging of the environment, and I'd think that it's less likely to involve slavery as well. If Thailand reins in the human rights abuses, their costs will probably go up, giving India and others even more of an opening. A market that works is a good thing for everyone!