I'd say it's more complicated than you're hinting at.
In this case, could it be at least in part a function of the likelihood that people will listen to and/or perform a piece of music after the copyright on that work has expired? Assuming such a thing is possible, of course.
To reduce the influence of external factors in the 'value' term. It just makes it easier to measure if one is purposefully factoring out marketing, branding and so forth.
It's fine to expect the sales of a work to go up if someone performs it well for a large enough audience, but is that measuring the value of the work, the performance, or the exposure? And basing the function on a period after copyright expires reduces the influence of stakeholders in the work who might pay for exposure.
I suppose. For me music is like fashion, it is in and of a moment. It is inspired by what came just before it, and it inspires what comes directly after it. Beyond that it takes on some sort of historical context.
