Really don't think shorter mission time would do much to improve the RTS place sales wise, or that it even has much of an effect.
I also don't think a reinvention along the lines of another game trying to gut out more staples of the genre, and dilute it down to an even more basic essence will work either. Plenty of studios have already tried that, and none of them have been as successful as Starcraft 2's iteration on Starcraft 1. Well I guess maybe you can stretch it, and say Clash of Clans does that, but I'd really hope for something less dumb.
I look more at something like Ultimate General: Gettysburg as a step in the right direction, where it fundamentally changes how you control units. Instead of placing constant points quickly, it instead lets you draw paths like a sports play diagram. Something I think is not only more accessible and natural for players to pick up, but since I've seen such a token few games even do it...just open for more innovation.
Quite a few newer RTSs in the late 2000s were focused on either reinventing core combat, or just sticking to the tried true rock paper scissors, but base building and resource management have received little attention to try to innovate. It's also the area that very much differentiates them over MOBAs, and if someone can make a new fun template for that along with some decent combat they have a real chance of getting attention.