I've already given my thoughts for why I think ICS won't be a factor, but I'll give some more thoughts on REX. It seems this game will offer a lot of stuff to do - especially early. Religion is a race you probably want to win. The same is true with trying to build culture on the civics tree or wonders or trade units (so you can get roads at a minimum) or military units. These are so many other things that settlers are a cost you'd have to bear. Given this, REX may be a strategy, but it certainly doesn't seem like the clearly best strategy. I'd much rather this (where opportunity cost is the limiting factor) than some artificial cost. That being said, limits to military conquest are still something that I feel is important.