Is declining movie theatre attendance a problem to be solved? Well, first of all, Hollywood continues to report record profits every year, so they probably have nothing to worry about for the forseeable future.
Declining cinema attendance is either a sign of change in the entertainment industry or an opportunity. Either you go with it and, basically, bank on streaming services for home use, or you produce a better cinema-centric experience that people can't get elsewhere. Not just a bigger screen and louder speakers, and definitely not 3D. Something fundamentally new to go in that space so recently vacated by all those cinema audiences. Think about what people will gleefully pay for in a public space like that, not what they will grudgingly pay for because it's the only way.
Mokalus of Borg
PS - The answer is also probably not "vaudeville" or "musical theatre".
PPS - If I knew the answer, I'd be on my way to doing it already.