"To promote the Progress of Science and useful Arts"
And the only reason nothing entered the public domain is that due to a copyright extension, there's a gap during which time nothing will enter the public domain. I don't like continual copyright extensions anymore than you do, but the mere fact of a single copyright extension doesn't illustrate that copyright isn't serving it's intended purpose.
Unless something changes, nothing will fall into the public domain again. And the act of repeatedly extending copyright was upheld by the US Supreme Court in Eldred v. Ashcroft.
As pointed out, more than one copyright extension has occurred. But it's more than just that: the copyrights are now automatic, and don't depend on something being labeled appropriately, as was true in the USA until 1978. That alone makes "public domain" a much less useful concept, as one is never quite sure what the status of something is upon inspection.
The term of copyright has not just gotten longer, it's gotten less determinate: author's life + some number of years. Instead of a simple date calculation, we have to go reference mortality data, too.
Those two things are a pretty massive change, making it difficult to tell if obscure works have entered the public domain or not.
"To promote the Progress of Science and useful Arts"
And the only reason nothing entered the public domain is that due to a copyright extension, there's a gap during which time nothing will enter the public domain. I don't like continual copyright extensions anymore than you do, but the mere fact of a single copyright extension doesn't illustrate that copyright isn't serving it's intended purpose.