What he said.
Actually, I used to wonder the same thing, albeit a little different... along the lines of, "If AIO is the base, SG x 2 above that, and something else above THAT (another sealant maybe), would the SG last forever?"
Of course, that's the same answer... with all the crap that your car puts up with in its task of getting you from place to place, there's nothing in the world that could last "forever" or even anywhere near that. Contaminants would eventually wear away the top sealant coat, and through the SG, through the AIO, through the clear, through the paint, through the primer, through the body panel, ... er, yeah, never mind.
There's also the whole thing w/ some of the stuff in P21S supposedly eating away Zaino if it's used as a topper on Z, but that's a little product-specific.
So... uh, yeah, what he said. :xyxthumbs