Only Congress can pull the plug for the U.S. Since Trump has totally alienated both sides of Congress, Congress will not pull the plug for him. Therefore, Trump's only option to get rid of NAFTA is if Canada and/or Mexico pulls the plug. This extension shows that is not going to happen any time soon.
This development will surely drive Trump nuts, as he is not known for his patience. If we're lucky, he'll have one of his tantrums on camera and maybe punch somebody, revealing his true colours for all to see.