I disagree
You have to keep both the gaming culture and popular culture in mind as co-existing but not the same thing.
If D&D stops being recognized as "the thing geeks do" by people who have never heard of Magic Missile, then the brand starts losing value fast.
The thing is, people who have never heard of Magic Missile have never heard of Pathfinder, either. Or WoD, or FATE, or any other tabletop RPG. If they're even aware that tabletop RPGs exist (thank you, Futurama / Big Bang Theory / Community), they lump them all together
as D&D.
Look at Transformers - was it a potential hit because people were really, really into the 2005-2007
Transformers: Cybertron cartoon, or because it was a big budget movie with robots punching each other and explosions that had big brand recognition from the 80s and early 90s?
They could shut down the RPG tomorrow and still make a successful D&D movie tomorrow. Agreed. But does it need the "D&D" name?
Let's imagine a spec script Universal has in their vault, about a ragtag team of medieval warriors fighting an evil sorcerer / dragon / demon-king / what-have-you. It's circulated amongst the producers and they like it, but don't know how it'll fare at the box office.
The question is, does attaching the name "Dungeons and Dragons" to that script make it more or less likely to succeed at the box office than developing it as a completely new IP? Especially if they're then able to add a Beholder fight scene and make the demon-king into Orcus?
That kind of brand recognition is
invaluable to movie companies and D&D has been a part of nerd culture for decades. As you've said, D&D has DEEP roots - it
could collapse as a game tomorrow, and no other brand would eclipse it for a very long time.
Again, if this is all that matters, why do 5E at all? What does 5E bring to the table that they didn't already have with 4E?
They want to maintain the brand value. If their value is $100 for movie rights and the second best brand is $3 for movie rights, then staying far and away the #1 isn't a concern. If two years from now their brand is worht $75 and #2 is worth $3.50, the fact that they are still vastly ahead of everyone else will not be the key issue. The fact that they lost 25% of the brand value will be.
I think you're drastically overvaluing how valuable gamer buzz towards 5E would be to a D&D movie. They could have easily ceased support of 4E and pursued the exact same brand strategy. They chose not to, in my opinion because it would have left the tabletop game on a controversial and sour note (the 3.5 / 4E split) and a "D&D Resurgent" narrative is much better press. Movies take a while to make, so even if 5E sales start tanking hard a year from now the studio will be already be producing it.
A recognizable brand name is the only thing that matters to the studio and the amount of nerds actually playing 5E in 2017 means diddly squat to them - and Hasbro will be able to show them some impressive sales numbers as recently as last year and spin them a yarn about the millions of former players from the 80s who'd probably go see it and that's all it'll take.
They made a movie out of
Battleship for crying out loud!
We have gotten pretty deep on a tangent.
The bottom line is that the more people actively playing D&D, the more success D&D will have at all levels. (the game itself and the overall brand)
You can assume that the D&D brand is immune to human nature and changing focuses over time.
You can assume that people playing D&D will just keep playing "just because".
I assume that maintaining focus on the game and keeping the niche hobby fan base from waning is better for the brand than letting the fans interest wane.
The more people actively playing D&D, the more success D&D will have at all levels - completely agreed.
However, people still play 1E. Hell, they play 2.E, 3E in original vanilla, .5 and Pathfinder flavors, 4E, Swords and Wizardry, Blood & Treasure, Labyrinth Lord, I could go on... none of those games receive the support 5E is receiving, but people still play them.
Could there be a mass exodus to the new shiny if 5E doesn't see more product come out? Sure, maybe, though I'd argue over timescale with you. But from a brand perspective, those departed fans are still just that - departed
fans. Are they playing 5E at the moment? No, but as long as they're not
anti all things D&D, they're still more likely than the average Joe to go see a D&D movie and therefore everything Hasbro and the movie studio has set out to achieve has been accomplished.
Playing just one game of D&D (any edition) with friends and deciding it doesn't suck is all it takes, even if they're not converted into lifelong tabletop gamers.