And all I can say is, I think MtG is going to at least try to make things better, the movie already has relatively low expectations (it's a D&D movie, those are always crap, doesn't matter that this one looks super shiny), and predicting that "One D&D" tanks is circular, presuming the game will fail and thus concluding that the game will fail.
Is this scenario possible? Certainly. Almost anything is possible.
Is this scenario plausible? I don't really think so.
I suspect the movie will be fair-to-middling. It won't start a brand-new D&D film franchise nor collect award nominations, but it won't be a box office bomb either. It will simply be a movie. The only way it would tank is if it has some kind of absolute, unquestioned, knock-your-socks-off unbeatable competition. That's an unpredictable event--you can never know how well competing films are going to do until after they're already out in the world.
I suspect MtG will take some hard knocks, and require a few years to recover, and may not ever fully recover. But it won't go absolutely, unequivocally belly-up, totally-lost-cause either. If it dies, it will be a long, slow death.
And then, as stated, any amount of presumption about "One D&D" doing poorly is circular reasoning. We must simply suspend judgment, and observe what happens. If "One D&D" truly does absolutely, unequivocally tank--if it crashes and burns outright--then that alone is enough to say "D&D has died" by the standards you've presented in the thread (since you classify 4e as a "death" despite former WotC staff saying it did just fine financially.) If it doesn't fail spectacularly...then I just don't see D&D "dying" even by the loose standard you've given us.