It seems that any language stating intentions in the Constitution has only as much relevance to real laws as the relative strength of the lobbyists arguing for/against the provision in question, e.g. "A well regulated militia..."

I'm not an expert on the science part, but as far as the arts are concerned, IP being inheritable not only does not promote progress, it actively harms it. See: The Marvin Gaye estate and the damage they wreak on the music industry by thrashing about every few years with another cash grab lawsuit based on hamfisted and incompetent explanations of music theory.

I don't think that is a very good example of active harm due to inheritance. Marvin gaye could have done the exact same thing if he was still alive.

Marvin Gaye was making music, which is to be encouraged. His estate makes nothing.

>or does it simply enrich heirs?

This was one of the original arguments in favor of copyright, except in reverse, as the poverty of heirs was an argument in favor of copyright:

When, in 1749, Thomas Newton's (1704-1782) edition of Paradise Lost was published, it carried details about the impoverished circumstances in which Elizabeth Foster, Milton's granddaughter, lived.[33] The account gave rise to a performance of Milton's Comus at the Theatre Royal, Drury Lane, from which Elizabeth received £130, the profits of the evening. When aligned with the fact that Milton himself received only £20 from Simmons, the tale became a staple in subsequent accounts of neglected genius and avaricious publishers.

Deazley, R. (2008) ‘Commentary on Milton's Contract 1667', in Primary Sources on Copyright (1450-1900), eds L. Bently & M. Kretschmer, www.copyrighthistory.org

I don't think it's grey at all, what other productive effort do people only do on the grounds that it will enrich their heirs? Maybe that's what it takes to encourage knocking off a king, if that's something you want to encourage, but I can hardly think of anything else.

Still that cuts in both directions, in that you could use it to argue that other properties don't necessarily need to be heritable.

Either way it's pretty clear the "limited times" they were imagining were a portion of a lifespan, not multiple lifespans. It's pretty ridiculous that as the world moves faster these durations are increasing rather than decreasing.

Could someone argue that transferability of copyright is not constitutional because it's not explicitly authorized by constitution ? The exclusive right is explicitly given to authors and inventors, not to other people.

I don't think you can argue that at all. The point is to promote the science and arts. Being and to transfer ownership makes it easier to promote the individual artists and inventor. Also how do you deal with large projects that have the efforts of a lot of people?