"... securing for limited Times to Authors and Inventors the exclusive Right...".
The weasels in Congress get around this by retroactively extending copyrights every time Mickey Mouse is about to enter the public domain. Such extensions are thefts from the public domain and destroyers of wealth, but the Supreme Court has ruled it constitutional.
The supreme court opinion was essentially putting congress on notice not to try this again. They weren't really going to farther.
If congress does it again, i'd expect them to be significantly more receptive to the argument that it is "unlimited copyright on the installment plan"
Yes, it grants congress the power to grant copyright protections for a "limited time." Although a limited time could conceivably be "any finite period of time," a set of which "1 million years" is a member, if one chose to interpret it that way.
1 million years would be pushing it, but there isn't a clear boundary. Eldred v. Ashcroft is the most authoritative case in U.S. law on this but it doesn't reach a clear conclusion. [1]
What's more interesting is the dissent in that case. According to Justice Breyer, the current copyright term is valued at 99.8% of a perpetual copyright (presumably because the present value of the future becomes increasingly discounted over time). Unfortunately, the rest of the court didn't go along with this as it would have implied throwing out plenty of prior copyright extensions as well.
