Well, any metric except producing product or novels etc but thank goodness its doing well on Facebook and Twitter.
Producing lots of content is NOT a metric for success. It's a metric for producing lots of content.
I can produce a hardcover game book every year, but that doesn't mean I'm a popular game publisher. There are tonnes of people producing two or three novels a year via Amazon, but they might not be considered successful novelists. (Heck, I
did write a
novel. So by the metric of producing content = success, I must therefore be a successful author.)
If producing lots and lots of game books was equivalent to being hugely successful, 3.5e wouldn't have died after five years and Pathfinder would still be the #1 RPG (instead of #2 by a wide, wide, wiiiide margin).
Plus, the current conclusions from the FR novel line thread was that WotC was wrapping things up to license to another publisher. Like how TOR is the licensed novel company for Pathfinder Tales. It's an easier transition if they don't have two or three novels on the go. After all, the issue with FR fiction was always getting in new, good authors and WotC isn't positioned to acquire skilled novelists, unlike a dedicated novel publishing house.
And/or they want the new novels to be tighter aligned with the movie(s). So they're waiting for that to be further along. Which makes some sense.
The FR novels have always brought in some money, being best sellers. The fact that WotC evidently thinks they can make
more money via a license partner - someone thinks it's worth that much to buy the rights - speaks highly of the success of the brand. They can get more money later so they don't need money right now.
So, ironically, the ending of the novels could actually be an indication of the strength and success of D&D right now.