Well it’s just the upgrade that they’re buying. So they’d just have the DLC without the game. Kinda like if you bought a game’s disc, then bought the upgrade, then sold the disc. Basically no access to anything that requires the game to load up.
I think it’s opposite with the PS3. They kept the option to buy PS3 games because of reactions. But it’s only possible through the PS3, so the PS3 games won’t clutter PSN.