Now, that might be what Toei would want out of a Decade movie where W just happened to wander through for a bit... but I've not yet gotten the impression Decade's popularity is on par with Den-O's.
Ratings wise, it was a bigger success than Den-O. Merchandising-wise, the Finalform ride toyline was popular enough to get a direct successor in Double's toyline. The movie was very successful too.
There's also the inherent crossover aspect of the series. Focusing on Decade also means focusing on references to the previous 9 Rider shows, at least, (but potentially more, considering All Riders and later episodes of the tv series).
I agree that just the Decade cast by itself isn't as popular as Den-O's, but, adding the crossover element, it seemingly becomes more popular than Den-O by itself, even without much participation of actors of previous series.
I just can't see Toei allowing W to be undermined out of the gate like that, though. This movie has been shot before there's any clear idea of what W's popularity level will be and it'll air quite early in W's episode run. Toei has every reason to want to use it to push the W property.
That's exactly the reason I think Toei won't give Double much focus. If the tv series isn't very successful, a Double movie, especially one made before they could gather reaction about the tv series, could be hurt by it.
Decade, however, is a known success which reverted the trend of decreasing ratings while also getting a successful toyline.
The movie exists to make money for itself, not to push a tv series. In fact, like Decade has recently showed, with its cliff hanger ending, Toei now seems to prefer using the tv series to push the movies.
Besides, giving Double little screen time doesn't mean undermining it. Just look at the Den-O&Kiva movie or Double in All Riders. Base Kiva seemed comparable to Climax/Liner Form Den-O and Double seemed stronger than everyone else in All Riders.
There's a high chance that promo was cut selectively to emphasize the Decade moments, given the context in which it was run.
Even if the Decade's elements had been emphatized, there's no way Double's elements could be completely avoided if it had some importance for this movie.
In this promo, we saw two new Decade-based antagonists: the 2nd Tsukasa and Kamen Rider Kivaara. Basically the entire main cast has been reconfirmed too.
Now, look at the Den-O&Kiva movie. Even if you cut only the Kiva scenes, you'd still basically only get Kiva facing a Den-O villain and interacting with Den-O characters. Same for the Den-O&Decade movie. Even the trailers that focused on Decade just showed him among many Den-O elements like the Denliner or the new Oni villains.