Disney is notorious for pushing for the extension of copyright lengths. Anybody that knows anything about intellectual property laws knows that IP rights were not meant to be indefinite. However, since copyrights were first legislated they were given a certain amount of time. Creators under that law were given that amount of time to profit from their creativity, and that's akin to a contract between the government and the creators. All this is fairly well known. What seems to get lost in the fuss is that there was also something akin to a contract between the government and the people, which is that limited amount of time.
When I entered into government employ, it was under a certain set of contractual obligations, both mine and my employer's. Of course, (ancient worker that I am) since then, terms of employment have changed, for example, retirement benefits. However, having entered employ under one set of rules, I can choose to retire by the rules I entered under rather than the new rules.
Further, for example, legislators can't make laws for past actions, and then go back and arrest people having committed those actions in the past, ex post facto.
So then, why is it that when new copyright legislation gets passed, all existing copyright holders can enjoy an extension in copyright term? Maybe the analogies aren't perfect, but I think it gets the point across. If I recall, when new copyright legislation is passed, it's worded with words like "100 years from the date of publishing" or some such, rather than "works from 1938 are protected for 100 years". Clearly, the wording of the latter concerns specific works, whereas the wording of the former is intended to be general. Either way, each time the term is extended, it is extended to cover specific works.
It seems to me that the only people pushing for copyright extensions are big companies; big companies built on creative works whose copyrights would have expired and passed into public domain long ago. If the system worked by my logic, even if they did push for copyright term extensions, it wouldn't affect existing works. That would really disincentivize such lobbying and benefit the public at large, I think.