I feel like D&D is stuck in this weird place where they have a lot of the elements to create a great media property, but the heart and soul of the RPG is in conflict with the key pieces that would be needed to grow it into a huge brand.
That's not an insurmountable problem. Imagine a movie where some ordinary guy from the real world gets accidentally kidnapped/summoned into a world of legend and then has to become a hero, a la Ash of Army of Darkness fame. You could make a fantastic movie with compelling characters and lots of Crowning Moments of Awesome, throw in some D&D-branded monsters (e.g. he fights a bulette in one scene), and brand the movie "Dungeons and Dragons" without ever using any 5E D&D conventions like "parties" or "homogenous party levels" or "balanced encounters". (In fact, balanced encounters are intrinsically boring and should be avoided in movies at all costs; arguably they should be avoided during play as well.) Maybe the plot of the movie wouldn't play out much like a typical RPG session, and maybe longsuffering grognards would have to spend years afterwards correcting everyone who thinks that "playing D&D" means actually running around with long swords and pretending to be Thok the Barbarian (who isn't even in most campaigns, they just made Thok up for the movie, dude!)... but it could still be a fantastic movie and drive the brand.
TLDR; a great movie which conflicts with the "heart and soul" of the RPG will cause Internet arguments but no real damage.
P.S. To be honest, I think the "heart and soul" of D&D pretty much requires the movie to transport a real-world character into the game world, because the heart and soul of the game is about players temporarily becoming PCs in another world. I won't say "escapism" or "wish fulfillment" although those are gateway experiences, but I do think that a movie that was all about Elminster doing stuff to Manshoon would be missing a central component of the D&D experience even if it captured the Forgotten Realms experience perfectly. (Is Manshoon still around? It's been a long time since I read those books...)
If you were to make a movie then the Dragonlance Trilogy would be your best bet. It jas everything you need for a proper movie.
Some things surprise us like John Carter, The Lone Ranger, and Green Lantern. I really didn't think any one of those could be a failure and the list could be much longer. I'm not sure what the answer is for how to make a successful D&D movie but it seems there are as many negatives as positives for each setting and known character in the D&D stable, not the least of which if you do one many folks are upset you skipped their favorite. It's a divided audience before you even put pen to paper, and that's your 'base' audience, the ones you would hope were already in your corner.
D&D movies, if done well, don't need to be nearly as iconic, but they need to be done well. I'm not certain Courtney Solomon can do anything well. We need to see stuff that can be reasonably replicated by the mechanics. In fact, being too faithful can be as bad as not faithful enough.
What did you think of D&D II and III?

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.