DKO

The foam is the vapor barrier on the outer walls. The foam can't touch the steel (potential corrosion). Tyvek wrap just insulates the foam from the steel and as stated in OP, the Tyvek allows exterior panel replacement someday without having to "sawzall" the foam apart. Foam will adhere to the steel.....forever. You can remove the sheet of steel from the outside & the Tyvek & foam should continue to stand by themselves. 2" of foam is R12 & vapor barrier simultaneously. Or at least that's how the plan was laid out to us today.