Then there's the economic value of a work: very few copyrighted works have an economic life of one year, let alone a century. This is particularly true since copyright protections were extended to all works rather than just those which were registered with a copyright office (this post is copyrighted, however my economic interest in it is nil, still, it will extend for the next 120 years).
The real argument for extensions is copyright term in works of enduring value whose copyright is about to expire. This is the "Mickey Mouse Copyright Law" in the US. Walt Disney's first Mickey Mouse cartoon, "Steamboat Willie" (itself borrowing very heavily from earlier works) was published in 1928, and there's been a strong trend in extension to prevent works from at least this date from ever entering the public domain.
This results in the exceptionally strong protections of copyright being extended to a huge volume of valueless (and often ownerless in the sense that no true owner can be identified or contacted) to protect a very small number of works of enduring economic value, though by no means does this promote the directive in the US Constitution "To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries" (other nations have their own justifications).
Initial copyright, until 1976, was for 28 years, registration required, with one renewal allowed for a total of 56 years (if I recall correctly).
While registration is still possible in the US (and is required for certain damages), it's not required. I agree that this would help greatly in addressing the matter of "orphan works".
These were never meant to be eternal, for good reason, and constantly extending them to me seems very corrupt... that this is favoring one group's interests over another.