It bugs me that there is never any accounting for the "loss" to the public. When a work that would otherwise enter the public domain is instead kept copyrighted, that is essentially privatisation of public property. If congress decided to sell national parks to private owners there would be a massive outcry. The idea that they would just give them to private owners for free would be considered utterly absurd. Yet this is exactly what extension of copyrights is. At very least, people would insist a proper valuation be put on them and the benefactors asked to pay a commensurate amount to the public to compensate the loss of public property. Yet none of this happens, these extensions are made as donations to private benefactors as if it means nothing to society.

This is because copyright law is unintuitive, and most don't understand its origins. People don't feel that creative works rightfully belong to the public. The existence of terms like "intellectual property" doesn't help.

Very much this. It is plain unhealthy how many people perceive copyright as being a 'measure to ensure income for artists'.

Hmm, at least in the US, the relevant sentence is: "To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries."

The way I read it, the intent of latter half of the sentence clearly is to enable artists to profit from their works. Note that it does not say "all artists shall be paid no matter what". But by having exclusive rights to their works, it can work roughly like any other property in a capitalistic society: that is, artists may be able to capture rewards proportionate to the amount of the value they provide.

I am not a legal scholar, but to me the intent of the latter half is clearly stated by the first half: "To promote the Progress of Science and useful Arts".

One way to promote scientists and creators is to allow them to warn money so they can keep sciencing and creating.

Nobody is arguing that. What people are arguing is that if copyright does not promote the progress of science and useful arts, it fails to fulfill the intent of its existence. The follow-on argument is then that indefinitely extending copyright undermines that promotion of progress. If you buy both arguments, then you see why many people find the extensions so objectionable on Constitutional grounds. I don't think many people disagree with the first argument, because it's right there in the text (which is the point I believe your parent was making), but of course lots of people disagree with the second argument, believing that longer copyright terms better promote creation.

That's obvious, but the more interesting discussion (which we're really here for) lies in the unstated part of the sentence: how were the actions stated in the second half intended to result in the goal stated in the first half?

Don't you think that authors who release their work freely under copyright believe that yes it actually is a 'measure to ensure income for artists?'

Or...maybe some payment to themselves for their work?