I would agree with you. I believe that multiple lore rich setting will balkanise the game to a certain extent that makes it a bad marketing strategy, no matter how much the fans of a given setting like that setting their purchases will not offset the losses from the people that will not buy because this book is Ebberon and not FR.
I notice that most third parties, that I am aware have one setting, if they have one at all.
This is one are where WoTC really needs the third parties and their support because across the third parties there can be a multiplicity of setting. Third parties are more stylistically distinct and for them that is a strength. It lets them stand out from the crowd.
This is why some of the complaints about setting content from WoTC being bland has merit. Their market is so large and diverse that a bolder product would be at risk of fragmenting their market. But white bread has its place, a lot of sandwiches start from white bread.