Magpul can and will leave if the law goes into effect. That much is clear. The question remains whether or not the economic impact of them leaving is enough of a deterrent to those pushing gun control laws.
Consider that the people pushing for stricter gun-control laws believe they have a moral imperative to do so. Do you back off on such principles because of a possible economic loss? Right now, a lot of politicians in Colorado believe that the wind is at their back, and that the loss of one tiny little gun parts company (well, that's what they're being told) won't make that much of a dent.
If Magpul leaves, good for them. They'll be in a better business climate, and they'll kept the moral high ground. However, I'm under no illusion that their actions will have much of a political effect.
Sometimes it’s nice not to destroy the world for a change.