>So far I have not heard any rational counterargument to that
Try this: when the artist signed on the dotted line, he did so on the terms of the contract in question, and within the legal provisions of the time. This latter included the promise that he would be paid every time someone made a copy.
The EU have chosen (probably correctly) to alter that context, so that a central pillar of his contract is diminished. In recognition of this fact, the EU simultaneously recommended 'comprensation' for that loss.
See - it's not that difficult after all, is it?