Life plus 70 is scarcely "nearly infinite", and since Disney didn't try to extend the Sonny Bono copyright extension so as to keep "Steamboat Willie" in copyright, this doesn't seem likely to change.
It's also the result of an international consensus:
Life plus 70 years means the things you grew up with will most likely be never released in your lifetime. It's really not that far from an infinite term in the effect it has.
It's also the result of an international consensus:
http://copyright.nova.edu/copyright-duration/