Parmandur
Book-Friend, he/him
Now might even be a fad, few years later its over.
5E I doubt won't last 20 odd years like some are speculating the only D&Ds to come close to that would be Basic which in effect had around 4 or 5 sub versions (Holmes, Moldvay, BECMI, Black Box etc) was technically around for 19 years (77-96 IIRC) but was more or less out of print/on life support for some of those years. 3.X if you count 3.0, 3.5, Pathfinder as the same system but I don't think we'll see WoTC drop the ball again.
Sooner or later the books will plateau but they can always go down the spam some splats path for a year or 3 or see how will campaign settings etc do. See what sales are like in year 6 and 8, its really to early and most sales of D&D come early in the editions lifetime and 5E is following that trend.
A D&D movie won't land for 2 years minimum even if they start filming in the next 6 months and D&D and good movies don't go hand in hand. The entire RPG market yearly sales would only pay around a quarter of a AAA+ game or block buster movies production values. That is the main reason you're not going to see a good D&D movie or game unless you get some cheap indie development that blows up.
Same reason most of the good D&D games date from the late 90's and early 2000's. Game development back then was around $4 million, now its in the $80-200 million range, at least for a decent big budget version. The number of 90%+ rated titles on the PS2 is another example.
People are getting carried away methinks with how big D&D actually is.
WotC doesn't foot the bill for the film production, Paramount Pictures does. The reason they are paying Habro for the privelage of making a toy commercial is because LotR was big, Game of Thrones is big, and there is potential money to be made.
Just because previous films have been bad, doesn't mean that they have to be bad.
Sales don't necessarily need to be book and bust, if an evergreen business model is adapted and new customers are groomed with things like early reader books, children's picture books, etc.
They don't need to resell everything to a small core group every few years, if they get new kids to start every year.