I am planning on using radiant heat in a multi-level concrete floor home, and am considering running a redundant, unused loop for the (very far) future in case it's needed. Crazy?

PEX is usually warrantied for 20-25 years, but what happens at that point if the tubing starts to disintegrate, systematically fail, or completely freeze over? Pour another shallow slab on top? Try to heat from below?

Having redundant loops of unused (dry) tubing that you could switch over to I think would be awesome. It could also be used if you spring a leak and don't have a full systematic failure.

Yeah the materials cost would be almost double, but frankly radiant tubing buried permanently in concrete scares me.

Would dry tubing last long enough before it needs to be used (say 25-50 years) or would it dry rot, etc?

Waste of time and resources.

I wouldn't worry about PEX as long as you buy a quality brand. I installed a 1-1/2" PEX water line back in about 1995 because it was subject to occasional freezing. I check it occasionally and it is as good as the day it went in. It even gets some UV exposure, which doesn't seem to be hurting it.

PEX isn't going to dry rot. It's an extensively crosslinked polymer. Pretty stable.