Also remember that markets change: what constitutes a niche market today may be mainstream in 5 years. 9 years ago, when I bought my first digital camera, its 8MP was considered large for its class, and there was a shot-to-shot lag writing to CF. As processors and memory get faster, and as user demand increases, eventually there will be a need for more MP. Demand may have hit a plateau after it achieved the speed/IQ/reliability to replace 35mm film. 18-22MP is more than adequate for most amateur and pro uses, and there are diminishing returns beyond that.
At some point, MP will creep up to meet slowly advancing demand and supporting tech. Yes, 35MP+ is a niche market now, but it won't stay that way forever.
From the business perspective, those companies who push out huge MP FF cameras not "innovative" they are desperate. Think of it: why would a tech company, any tech company, blow all its best tech on a current generation product? They'd want to hold some back for the next gen. And who wants a mutant camera with a great sensor, but second- or third-rate components and "fit and finish?" Some, yes, but not most. Most people want something like a 5d3, where everything just works.
I don't mean to disparage the D800, which seems to be a great camera, and I really hope the Sonys turn out well because competition is good and I'm a fan of EVF; but yes, these are currently niche cameras put out by companies desperate to improve their market position. As long as Canon (any company) leads in market share, you can expect them to trail slightly in innovation because innovation is inefficient.