As the title says. D&D has almost died 3 times but could it happen a 4th time?
Sure. It doesn't seem likely right now, but then the current levels of success didn't seem likely a decade ago.
Here's my scenario.
1. The D&D movie flops (for whatever reason).
I don't actually think a simple flop affects the game all that much. However, if it is a spectacular failure, and ends up regarded as an offence to cinema (alongside such wonders as "Highlander 2", "Star Trek 5", and "Dungeons & Dragons"), that might have the effect of tainting the wider brand.
2. One D&D flops for whatever reason.
This is a tricky one, since failure is relative, and we don't know Hasbro's expectations. Given how well D&D has been doing in recent years, though, those could well be quite high.
3. Hasbro/WotC themselves get into trouble due to whatever reasons. This means no MtG money to bail out D&D.
Funnily enough, we're probably here already. But it's just a bad time; this too shall pass.
But you're right that the combination of D&D doing really badly at the same time as Hasbro doing really badly could see it getting shelved. When things look bad, big companies have a strong tendency to pull back to focus on their "core brands". Right now, D&D is almost certainly one of those, so will be fine. But if the wheels did come off at just the wrong time...
Even then, all that refers to D&D as an ongoing concern, and D&D by name. Because of the OGL, there will always be near-D&D games (including a 5e clone if the worst happened), so the spirit would live on. And, anyway, it's not like D&D stopped being played even when it was achingly uncool.