I strongly suspect that the Forgotten Realms would not have been worth much of anything as an IP but for its use in Dungeons and Dragons. Rather generic fantasy settings (which is what it is, that's what I like about it for the record) are a dime a dozen, and TSR would not have invested heavily in one they didn't own outright when they easily could have just used another of the ones they owned outright (so even some sort of ongoing royalties arrangement was not realistically in the cards).
If Greenwood was an exceptionally talented author or the owner of his own popular rpg, or computer game, or other product in need of a fantasy setting he might have been able to turn a rather bog-standard fantasy setting into something he himself could see more substantial profits off of. But I think in the scenario where he, as the person he is, held on to his IP he would have maybe sold a few obscure fantasy novels.
Meanwhile by selling out Greenwood has gotten a prominence, prestige, and at least some degree of employment over the years. I just don't see the realistic scenario where that particular person held on to that particular IP and it worked out the same or better. I'm sure the rights holders could have been a bit nicer to him over the years, but he's hardly been a victim.
