"Metaplot" is a...complex...topic, but essentially what we are talking about WotC deciding not to do: have an internally consistant ongoing canon set of stories, that crossover from RPG modules to books and other media, such as the Time of Troubles in FR. Earliest example I suppose would be the DL modules, but in 2E that really became the going thing.
Thanks. I can see why WoTC would make this decision. The books, movies, and comics should try to have some canon consistency, but the campaign materials for the RPG need to more skeletal so DMs can flesh it out as they see fit without worrying about being inconsistent with canon.
I've never read any books set in the FR, but it would bother me if the books were inconsistent. It ruins my suspension of disbelief. I treat the settings books like SCAG and the APs as seaparate from the book---easy for me to do because I never read the books. If I were a huge fan of the books, then I should be able to fill in the details of my campaign with canon, but i don't think it make much business sense for WoTC to spend a lot of time developing those canonical details.
And, again, to avoid getting into a flame war that I have no stake in, I don't know that the FR in 5e is inconsistent with canon or that WoTC doesn't care about canon (they sure talk alot about the history of the Realms going through decades of material on Lore You Should Know). I also have to special insight into the RPG industry. But what I have observed is:
* I've seen more complaints from people who do not like the FR at all and want different settings
* Most people are fine with the FR as the default setting and it is rare to see people upset about issues of canon
* Those who really care about FR canon seem to be small minority
* 5th Edition has been a great success for WoTC
They seem to have a formula that works, at least for now.
Assuming the D&D movie is set in the realms, if they don't screw it up, if it is a success, I expect we'll get lots more material about the Realms, much of it marketed for general audience who don't play D&D. This will be good for ensuring further development of the realms, but, again, I'm not sure that staying true to canon will be the top priority.