Hahaha! Well, from one perspective I can see it as super shallow, too, and that makes me laugh. "new marriage, new diamond. Duh!". LOL Oh, that cracks me up.
However, on the other side of things I can kind of see new rings as a whole new start. I've even considered it myself as something I'd want. I don't care if it's just a silver band for both of us, it's not the cost, it's the representation, the visible reminder of what we've worked for and that it's new and different and whole.
Maybe you're mind reading into thinking she wants a 6 ct Harry Winston? I don't know. Try not to mind read in that case because in the end, the prize will be the marriage, the rings (new or old) will just be a representation of that and that's not really something to worry about right now.