@AC - you're not much of an advert for copyright extension...
The problem with that theory is that copyright was originally introduced as a trade-off, well after the initial establishment of copyright as a legal concept - allowing creators & publishers the exclusive right to exploit their work for a defined period of time (and providing them with the ability to legally pursue anyone who infringed those rights), but also defining a point at which those rights ended and their work became part of the public domain.
What we've seen since is a shedload of people, some of them very well rewarded and some of them utterly abused and mistreated, wanting to change copyright and legal conditions to better suit themselves. There has to be some element of adhering to the original contract, even if the unexpected success of your creation means that you later realise you could've made even more money from it.
It's particularly suspect that it's generally publishers rather than artists or creators who start off the "ooh, copyright should last longer" - of *course* they want that, they *love* the idea of having exclusive rights to a given product for ever-longer periods, given that the cost of production is frontloaded.
The chances of people paying for a given cultural item if it is neither new (where new is, say, less than 1 year old) nor particularly well-regarded (where positive regard is subject to attrition as time passes) are inversely proportional to the amount of time passed since its original publication. Continually extending copyright to protect the ability of content producers to make more money from the same content is a disservice to society as a whole, because it directly undermines the public domain concept under which copyright was originally introduced. Essentially, rights-holders threatening to take their ball and go home.
And, well, fuck 'em, or at least fuck the ones who aren't willing to adjust to changing usage patterns, behaviours and technological landscapes. My expenditure on books, films, music and games is quite high now that I have disposable income available, but the only reason I have a desire for these things is that when I was younger I availed of pirated versions - because there was absolutely knack all (well, with the exception of poor-quality classical music performances) available in the public domain.
If said pirated versions hadn't been around and there hadn't been decent libraries either (which there feckin' won't be, given what our Tory Overlords are doing), I just wouldn't have developed an interest in those things and thus wouldn't be spending my money on the output of those industries.
TL;DR - rights come with conditions and obligations attached.