I get what the OP is saying. I've had the same situation before where I'd like to use a GUC but can't book into PS. I'd be happy to pay the PS price and select a seat in Comfort plus in order to be able to use a GUC to upgrade to D1. Depending on the price, I'd do this to waitlist but It's a no brainer if there was inventory availability in D1. Delta could be missing out on some revenue in a case like this. You'd think that this situation probably doesn't happen that often, but I feel like it's happened to me twice now.