The core problem here is that adhering to Berne is an obligation of TRIPS, and adhering to TRIPS is a condition of WTO membership. Theoretically, WTO conditions can be renegotiated; in practice, that is a huge pain in the rear, given nearly 200 countries have to all be brought into agreement.
In addition to that, the US is party to a whole host of other trade treaties that have IP provisions specify minimum terms of either life +50 or life +70. In principle these could all be renegotiated, sure . . . but in practice?
Blowing up the whole framework of modern international trade by unilaterally abrogating all those treaties is something that the US could do, sure. But absolutely every US company that exports anything -- and their workers -- will flood the offices of Congress with noise if a bill that would do so ever gets anywhere, because it'll threaten their livelihoods.
So, existing copyright terms are not going to be reduced (Berne prohibits it), future copyrights will not be for any duration less than life +50 years (Berne/TRIPS/WTO and a whole pile of other trade agreements), and future copyrights will most likely be life +70 years (additional trade agreements).