How about temporarily halting copy rights for, say, 5 or 10 years? Then evaluate whether it's been good or bad for the production of cultural artifacts?
It's not necessarily untenable, but it would have some very painful consequences.
Something more like: For the next 5 years, all copyrights before 1970 become subject to the "author's lifetime" standard, and nothing else changes. If the experiment fails, then those rights are automatically restored to any corporations/estates that owned them. (With some mechanism to shield copies made in that interim from future litigation.)
That sounds like a more balanced version of the experiment, and I still think it has very little chance of coming to pass. I'm not good with Bayesian formalism, but my priors are something like: status quo most likely, permanent (but small) concessions to get a little bit more old stuff into the public domain next most likely, all other options highly improbable.
Finally: maybe it's sensible to try to shift the Overton window by calling for the abolition of copyright. But I don't know how one does that kind of thing ethically if you don't really want to go that extreme.