That's part of the issue. Many people knew it was bound to happen, but when you make a game like this; a game that's got all the flash, the pretty environments, the toned down combat, and all that work put into some of the animations, without the difficulty or skill ceiling of previous titles, you're going to get a game that makes a splash, and fades quickly.
The issue is twofold. One, the game didn't make as big of a splash as was expected, and, and two, how many people care anymore? Most people have moved onto the next shiny new object that catches their attention, and you're left with a game that was highly praised, and ultimately forgotten. Yes I suppose it could be brought back to relevance if all these other plans pan out, but then what? A year later, and it will be in the same position.
The turnaround time for games like these is too high to try and follow in the mold of some other series. A new DMC's not going to be released on a yearly basis, so if you want it to stay in people's minds you really have to give them a game they can play for more than a week. There's got to be incentive for the player to stick with it, keep it in the public's mind so that two or three years down the line your next game will hopefully outsell your last one, growing the franchise, and in an ideal world making fans and investors happy.
"Showing off is the fool's idea of glory" ~ Bruce Lee