So is this the beginning of the end of medium format?
No.
I'm pretty sure that medium format sensors with more than 80MP area already under development, if not cameras using them.
I'd like to rant here about the purported relationship between pixel size and image quality, especially given the recent story by Mark Dubovonoy but there's already been enough said about that bit of
fiction. I'll just say that the IQ180's sensor has a pixel size smaller than that of the 5D Mark II and thus also the 1DX, so clearly both of those cameras must therefore have higher signal to noise than the IQ180 and so on.
There's a constant push for "better" and "more" and this will continue to drive sensor development for the foreseeable future because if you don't then the guy next door will. And why will he? Because it is both a marketing advantage and because customers want more.
A medium format sensor using the same pixel density as the Nikon D800 would be 91MP.