No, I don't think it's that dumb. That's actually exactly what I stated above when I said that I have witnessed both AA and AS shift F/J to Y inventory in the case of Y oversells, essentially creating a scenario where there will be guaranteed upgrades for some passengers who paid for Y fares.

I think your point of contention was whether the inventory would still be available for F/J purchases, and, while I agree it would seemingly make more sense to ensure both premium cabin and coach seats remain available for purchase, I said I've seen plenty of instances on those airlines where that has not been the case... in other words, they zero out F/J and rely on the prospect of the Y inventory being purchased.