I'm not a lawyer, and I don't know. However, at present I understand there is a set date, absent further legislative action.
it's plausible, though doubtful, that extending the copyright given to new works will provide additional incentives for people to produce them. I can see no even-remotely-plausible way that extending the copyrights on already-existing works will have such an effect.Can you suggest any?
The reasons that have been given are
The Justices Scalia and Thomas have showed far more ability to define the more ambiguous term "reasonable" with regard to searches than with something far more amenable to definition as "limited Times".
"to encourage continued development of already-created works."
By restricting who can engage in such development? Note that since substantial improvements of existing works are protected by copyright, this extension was not necessary to provide such encouragement for the existing copyright holder.
"to strengthen the United States balance of payments. "(presumbly very indirectly via improving the national well-being)
By ensuring that materials will be copied freely abroad for many years before anyone can use them domestically!?
"To harmonize international copyright laws" (presumably differing national standards weaken the overall effect of copyright, and thence lessen incentives. )
By passing copyright laws which are longer than those in other countries?
"extended protection for existing works will provide added income with which to subsidize the creation of new works."
Anyone who wrote something 70 years ago and hasn't written anything else marketable since isn't apt to use their income from their 70-year-old work to subsidize new works.
Most works are effectively abandoned by their copyright holder within a decade. A 28-year original copyright term, renewable to 56, allowed copyright holders who were actively receiving value from their work (and thus producing value with it) to be protected for over half a century, while allowing works which were ignored by their creators to be fodder for those who would use them better.
Of the claimed reasons above, only the last is even in the foggiest bit plausible, and it highly strains credulity.