Petri,
I think it has been proven a few times that physical fate sharing is only a minor contributor to the total connectivity availability while system complexity mostly controlled by software written and operated by imperfect humans contribute a major share to end-to-end availability.
Yes, and at the very least would seem to match our intuition and experience.
From this, it can be deduced that reducing unneccessary system complexity and shortening the strings of pearls that make up the system contribute to better availablity and resiliency of the system. Diversity works both ways in this equation. It lessens the probablity of same failure hitting majority of your boxes but at the same time increases the knowledge needed to understand and maintain the whole system.
No doubt. However, the problem is: What constitutes "unnecessary system complexity"? A designed system's robustness comes in part from its complexity. So its not that complexity is inherently bad; rather, it is just that you wind up with extreme sensitivity to outlying events which is exhibited by catastrophic cascading failures if you push a system's complexity past some point; these are the so-called "robust yet fragile" systems (think NE power outage). BTW, the extreme sensitivity to outlying events/catastrophic cascading failures property is a signature of class of dynamic systems of which we believe the Internet is an example; unfortunately, the machinery we currently have (in dynamical systems theory) isn't yet mature enough to provide us with engineering rules.
I would vote for the KISS principle if in doubt.
Truly. See RFC 3439 and/or http://www.1-4-5.net/~dmm/complexity_and_the_internet. I also said a few words about this topic at NANOG26 where we has a panel on this topic (my slides on http://www.maoz.com/~dmm/NANOG26/complexity_panel). Dave