My point is that the most logical way to appease the Realms fans is to roll back to 1375 DR. You're only "invalidating" a couple of game products.
Don't forget those moneymaking novels you mentioned.
I see your point, but I don't agree. Further, I think older fans of the Realms are looser in their views.
If the OP will allow us the privilege of conjecture vis-a-vis the Sundering's purpose, then I think WotC is working to create a post-Sundering Realms that's very much the feel and flavor of the late 1300s, but set in the late 1400s, with the goal of capturing a new audience.
Certainly there will be enough there to make older fans happy, but that minor portion of the Realms fanbase consisting of older fans who adamantly insist on a leap back in time won't be happy, and so won't come along.
And that's OK, because WotC already lost them five going on six ago.
The real impetus to continue advancing the timeline past 1485 DR or so was the fact that the novels provide more profit than the TTRPG.
I don't doubt WotC likes to make money, just as I'm certain the relative few Realms sourcebooks introduced in the 4E era don't outweigh the novels in terms of sales.
If both the 4E sourcebooks and novels were selling poorly, I think WotC would've taken a hard look at jumping back in time.
AKAIK the novels are doing well enough. I hear the Neverwinter Campaign Setting book sold pretty good (not sure about the Undermountain and Menzoberanzan books).
However I'm pretty sure that if WotC fired a peremptory shot across the bow of the designers/authors by saying, "We need you to fix the Realms, but we want you to do it by going back in time," I'm certain the designers/authors would've balked, if not walked out.
I get the strong sense they value continuity above all else.