Consider the question asked then.
At which point do you think a campaign is no longer using the Forgotten Realms?
Having never played a Realms game...
I'd say when the vast majority of the NPCs and organizations do not exist or are drastically changed from their original state. This includes Gods and Churches, Thieves Guilds and Armies, Cults and Sages.
If I talk to a DM and he has no idea who the Harpers or the Zhentarim are, and declares they do not exist, it is not FR, because those two groups are massive forces in the world from what I understand. (I know the Zhents because I watch Acquisitions Incorporated).
Which at least means the publisher has stopped putting out contradictory information or changing things with every edition...
Which helps us new players how? Other than trolling online stores for PDF copies of things that might still exist... how are we supposed to use these settings? Just declare "This is Greyhawk" and run an adventure set in England? And sure, PDFs are nice, very portable, but then we have to convert a lot of material and hope we purchase the right product that gives us the information we need.
For those lamenting the FR-ness of the current edition of D&D - what's an alternative that's going to make sense from WotC's perspective of needing to make money with their publications? Certainly not supporting a plethora of alternative campaign settings. That's part of what helped burn TSR. More generic alternatives may work, but adventures work best and read better (as far as I'm concerned) if they're fit within a context already. And from WotC's perspective, I'll bet they're more fun to write if they're written within a particular setting context as well. Plus, I figure that setting-based adventures and setting materials encourage sales of each other.
So if you don't like FR-focused adventures, I suggest you resign yourself to get used to them unless WotC decides to shift completely to another setting (and then Iuz help you if you don't like that setting either).
How much support is really needed though to give people options?
Two books. Give people the Sword Coast Adventurers guide for Eberron and Dark Sun, and now we have three vastly different game worlds to play in. Then, further down the line, maybe an adventure takes place in Eberron, with a blurb about how to use it in FR if people want. Then, a year or so later, set an adventure in Dark Sun, with a blurb about how to use it in FR.
We then have 3 fairly equally supported worlds, and "the default setting is the DnD multiverse" is actually a supported statement.
I get that such a decision would mean stopping support for FR for the next few years, but that shouldn't kill the game. Even give us one additional setting, and there will at least be something else to choose from. And I don't think it will be a serious financial bomb, I think such a product would actually do very well, because there is a craving for new things.