Any layoffs at Michoud would be temporary since it needed to build the upper stage of the Ares I rocket and the core of the Ares V booster - which is built around a new version of the Shuttle's external tank. The jogs and assembly lines would most likely be mothballed until the prototype Vs start rolling. So there's no reason why the plant couldn't be kept going if the Shuttle had to be used for a little longer.
The most pressing problem for the Shuttle fleet is that Atlantis' various pressure vessels used to store nitrogen and helium are life-expired and cannot be replaced within the current lifetime of the ships. NASA originally intended to cannibalise Atlantis to keep Discovery and Endeavour flying, but I see they've recently decided to keep Atlantis going for as long as possible. Any major failure with Atlantis that meant it couldn't be retained in service would cause huge hassle for the US.