Dear Antoine,
Thanks you very much for your kind and thoughtful reply.
It would indeed be nice to have more opportunities to discus this and other issues.
Let me just react here to one of your statements.:
" I remain convinced that the price to pay to have fundamental non-linearity is much higher than people think. Nicolas Gisin's formulation of the no-go in terms of faster than light signaling is probably the most impressive, but in the end I do not think faster than light signaling is the main issue. In essence, I believe the problem is more one of predictability, and ability to separate systems into subsystems for all practical purposes. "
I agree with you that linearity makes things manageable as far as our ability to analyze things is concerned. In particular as you note, the for all practical purposes' ( FAPP) separability is extremely convenient. But why should physics be that way. We already know (as shown for instance in Bell's theorem and related results ) that locality, a premise that seems well tied to separability, is not a fundamental feature of nature. It took us humans a long time to come to terms with that Why should it be so at the practical level in general. In fact, it seems to me that if things in nature were so, we would have to consider some kind of fundamental conspiracy. The world is nonlocal, there is no separability but nonetheless the laws of nature are so as to hide to a dramatic a universal extent those facts from us.
Does it not seem more natural to think that we happen to live in a region of the universe where that separability works at the FAPP to a very large extent, simply because is only under such conditions that life might evolve. Thus we would be deceiving ourselves by a clearly understandable but contingent condition of our immediate environment rather that facing the fundamental realities of nature. No conspiracies there.
Looking forward to seeing you somewhere and to continue our exchenge.
In the meanwhile all the best,
Daniel