IMHO the only thing stopping an extension is that next year is a mid-term election year and a lot of house members will be busy. I have zero doubt that this congress would not push back against a major corporation like Disney on a matter like this. The best hope for saving the public domain is general dysfunction on the hill. This is especially true if you get people like Lawrence Lessig in there arguing against extension, some members would vote for it just to stick it to him and all of his ivory tower leftist friends.

They always seem to find time for amending the copyright law, sometimes multiple times in the same year. A list of amendments is at https://www.copyright.gov/title17/92preface.html. I'm not sure what has happened since 2010 though, since there's only one change, in 2014.