I'm actually willing to accept the argument that copyright term should be independent of the life expectancy of the author -- that does affect the perceived present-value of the work. It's also how copyright was initially construed: a 14 year term, extensible once. Later extended to 28 years, with one renewal (by the author or heirs), to a maximum of 54 years.

What I don't support is for the term to be either indeterminate (as the repeated extensions of the past century have established), or perpetual, or even particularly extended. There are only a small number of cases where a 50-60 year term wouldn't be sufficient, and the concept of an extension means that unvalued works would enter into the public domain quickly.

Then we are in agreement on the first part; and I have no opinion re. the latter.

Re: the latter. Look at the Constitutional basis for copyright and patent, at least in the US.

To achieve the ends, that is, the progress of science and useful arts, the means is provided: exclusive rights for limited times.