Does it require steel rebar or could you use glass rebar? I think most homes (that are "boxes") have concrete in compression, and not tension. That means the structural considerations for rebar are a lot lower. I think, though, as always, the lifetime of a structure is always dependent on its roof. I don't think I've seen any "multicentury" solution for roofs. Even a lead roof is only definitely good for about a century before it needs maintenance; although, it's useful for twice that long. I've got a metal roof on my house, and it came with a 100 year warranty. (The UV eventually gets to the paint, which will take out the roof.)
Unfortunately you need to spend some time understanding loading on structures to understand what reinforcing is doing. Reinforcing is strictly a tension reinforcement - if the loading in compression is too great for concrete the solution is not to reinforce the concrete, it's to use a different material like steel. Concrete in pure compression is better off with no rebar at all.
Just because things fall under gravity doesn't mean they only experience compression. A simple moment frame would create tension in vertical members with only a downward load on the crossbeam creating moment.
It also would exclude the ability to do crack control in concrete with reinforcing, which would greatly lower its lifespan.
Water could reach the rebar from the ground, but I would think waterproof membranes and proper drainage should allow it to remain mostly dry. The foam panels should also reduce the amount of air that gets to the concrete and the rebar. Of course, you want concrete to be wet for curing, but how would water and oxygen reach it after it drys out? Do you think the oxidization process will not stop after curing?
concrete is porous. IF you wrapped your concrete in a waterproof membrane, the first tear or piercing in the membrane would then become an entryway for water and there would be no way for the water to leave because of the rest of the membrane. Proper drainage is a better solution.
It's like the bridges that used epoxy coated rebar for chloride corrosion resistance and then found out that any tiny nick in the epoxy coating then causes that rebar to somehow corrode FASTER than uncoated rebar.