That's quite a claim. It's conceivable that someone today considering whether to devote time to producing some work of art, rather than pursing another endeavor, would make a different choice depending on how many years he might expect that he and his heirs would have exclusive rights to it.
You went from an unprovable absolute claim to providing anecdotal evidence. I'm not saying you're wrong... just that what you've said so far doesn't convince me that "copyright should not be effectively forever" or that "50 years after they are dead" is "effectively forever".
This is by no means my area of expertise, but I'll give you an (probably unoriginal) example. Let's say an artist creates something at age 30. He could use some extra money at age 70. Wouldn't he be able to sell the rights for more to someone who the law will allow to retain those rights for more years after this artist has died?
The only question is would having long copyright have motivated Picasso to have created more art when he was alive. Given that the extensions to his copyright occurred after he died we can be pretty certain that unless he was a clairvoyant that the later retrospective changes to the copyright laws did not motivate him to produce anything.
More fundamentally great artists create art because they have to - it is part of their core. Art existed before copyright for exactly this reason.
I actually buy a lot of art - I have long since run out of wall space to hang it all :) None of the artists I buy are producing art just for the money and copyright plays no role in why they are an artist.
You seem to be presuming to know the motivations of every artist to have existed based on your personal experience. Is it so inconceivable to you that there may be someone who, let's say, is considering writing a book, wouldn't at all be motivated by how much money he could earn in return for the investment of his time? And take into account the fact that he could potentially sell the rights for considerably more, the longer copyright guarantees exclusive rights to the owner?
The only value to retrospective copyright extension is to give a windfall profit to rights holders long after the artist is dead. It is outright theft of our cultural heritage.
That is not what you originally said. You said:
> Not one new artwork is produced by still allowing copyright on Picasso's artwork.
I pointed out that the same copyright terms which apply to Picasso's works may motivate some artist, somewhere, to create "one new artwork" which he wouldn't have otherwise.