I remember you going through this. Actually, I tried to find that returned stapler in the Amazon Warehouse but someone else had already scooped it up.
I think the cracking is from the plastic aging and the unsupported areas failing. The plastic offgases as it ages and I suspect the dark areas around the cracks of yours was from exposure. I suggested Wade trying fix his since it was the first crack and (I believe) his is parked inside. So, I'm under the impression that his plastic is in much better shape, molecularly. But, with the type of plastic that it is, it will continue to get brittle. Ideal conditions (minimal exposure to harsh vibrations, UV, temperature extremes, etc.) slow down the decay, but it'll still eventually happen.
The cracks I epoxied have stayed just as they were almost four years later. But, I've got plenty of newer ones that make it not worth trying to fix any more. I still have my new dash in the shipping box. I guess I'll put it back on the '08's radar since it doesn't look like I'm getting another Tahoe.