The reality is, the post-Spellplague Realms are too far past where the true experts in Realmslore are really at. If you go to Candlekeep you will see that all the genuine experts there are still in the 1370s era, Ed included.
Hold on a second: not all that you're saying here is true.
If you go to Candlekeep today and
ask Ed Greenwood a question about the post-Spellplague Realms, you might just get an answer about the post-Spellplague Realms
the very same day.
The same can be said if you ask Brian Cortijo a question at Candlekeep.
That, and Ed's been writing extensively about the post-Spellplague Realms since 4E was released, in both novels, regular Dragon and Dungeon articles, free articles on the WotC website, and at Candlekeep. That’s years of work.
Yes, some lore lords of the Realms have stuck to the pre-Spellplague Realms. For example, George Krashos just gifted the scribes of Candlekeep with a
High History of Impiltur (it's a fantastic read, too), and he's said he isn't looking to go past the Spellplague, because the
era of time he's interested in is what he considers the sweet spot.
So yeah, some are sticking to the pre-Plague Realms, but some are not.
Whatever support FR is going to receive beyond Paizo-inspired adventure paths a couple of times a year is simply not going to be able to compete with the sheer volume of existing lore and existing campaign and adventure ideas that you can find in places such as Candlekeep and, of course, all the FR products released before 4E.
First, I write stuff for the post-Spellplague Realms and post it to Candlekeep. I also collect Realmslore in my areas of interest, and I include the post-Spellplague Realmslore. Other people are doing the same with their regions of interest.
Second, I don’t think we’re there yet in terms of writing off the production of new Realmslore, because WotC hasn’t even released a 5E Realms campaign guide yet. Once they do that, and once they start producing sourcebooks, and once we have an idea about how well the recently released adventures have done, then we can gauge people’s interest in the new material.
I see your point about the availability of older material, but in looking at the responses of people on this and the other FR thread here at EN World what I see is people saying “I already have all the material I need” and not “I’m new to the setting but I’m only going to use the old stuff.” New people are coming to D&D all the time. The new Realms may just grab their attention and keep them.
RAS and his young fans get what they want and everyone else gets a FR with tonnes of support today.
I'm a fan of Salvatore too, and I turned forty this year, so not every fan is a "young" fan.
Granted I don't buy all of his novels, but lately he's been hitting one home run after another. Some of his books I have dog-eared and filled with bookmarks, because there are passages in them worth reading again and again.
There's no denying his (positive, in my not so humble opinion) influence on the Realms.
Not looking to get all fanboy on you here; just trying to demonstrate that pigeonholing authors doesn’t give an accurate reflection of what’s going on and who their fanbase is.