Lot's of good stuff, that I will respond to in length when I get a minute. Thanks guys!

On the basic level, I keep thinking that no enclosure or building method is going to last forever, that is a given.
How long will SIPS last? That question is hard to answer, but lets be really generous here and say 100 or even 200 years if detailed properly (which I know is unrealistic).
That means that in a mere hundred years all that foam is going to end up...where???
I don't know that I want that the guilt of that on my conscience.

In the end, I can imagine clay and wood chips or even wood and mineral wool making a pretty graceful return. Foam, not so much. Sips may not be forever, but the foam is.