Is the FX revolution fizzling?

The simple fact is people like you who would pay a premium for a pro DX body are a very small segment of the market.   That is the issue not the merits of DX for the uses you describe.  There simply wouldn't be enough takers at the price Nikon would have to charge for a full pro bodied D300 successor to make sense.   You wouldn't choose the D600 even if it was just couple hundred more than the pro DX camera but the vast vast majority of the market would.