Also, Forgotten Realms is valuable IP. By now it's about as rich in lore as Tolkien's Middle Earth, due to all the content creators. And having Ed Greenwood as custodian is very smart.
This is a BIG deal that most people overlook. A GREAT RPG is about the story and the lore more than the mechanics. They could have done DnD 5e, in my opinion, and if they had not used the Forgotten Realms the success would not have been as great.
