I don't think Ed got screwed on that deal, as it's not like that was all he ever got paid. TSR paid $5G for the initial rights to the Realms, and in particular the contents of that first grey box, but they also kept paying him for new Realms-related product he created, including royalties on the novels (if not the game books), substantial enough that one of his royalty payments would have pushed TSR over the edge into bankruptcy at a vulnerable time. (And I presume, with TSR having been bought out by Wizards instead of becoming insolvent, his royalty was eventually paid in full. He didn't surrender that royalty in exchange for the return-of-stale-rights clause, he just agreed not to push for timely payment at a bad time.)
They also gave Ed further payments (it's implied) when they made more extensive use of some of his Realms creations, such as the character Elminster. And of course any new characters, places, or other Realms info he created AFTER the grey box would have been new material subject to later rights agreements as well as authorship payment and possibly royalties. (Since RPG game sales have not been publicized the way novel sales figures are, it's likely that royalties were never a part on that end). It's more like the initial $5G just cleared the way for TSR to be confident they had enough rights over FR IP to commit to a major investment in related product development with their in-house developers as well as Ed.
Given how notoriously bad TSR was at actually showing a profit on their huge sales, it's also not clear that he wasn't getting his "share" of what they were making. At least he didn't have to take part in their losses!
(He didn't mention it, and it wasn't asked, but I wonder if he also got something from Wizards when they were firming up the rights to D&D IP at the time they made big lump-sum payments to Gygax and Arneson to forestall any future disputes. It sounds like Wizards was in a mood to make sure they had clear ownership of TSR's key legacy product lines to the satisfaction of the original creators and perhaps that included Ed's FR.)