Me being an amateur does not mean that I lack the ability to observe, and I both observe and discuss with professional photgraphers. I dare to say that the client that count pixels is a minority, but I'm sure they exist and obviously you meet them, so you probably need MFD then and the clients can surely pay for that cost.
However for most it's about general printing needs, and those printing needs have not changed much since the best MF were at 22 megapixels not many years ago.
As an example I know of a very successful Swedish architectural photographer that has MFD tech cam (Sinar Artec), but it sits on a shelf as the D800 with tilt-shift lenses reached the quality level needed for his professional work and he found the workflow to be more convenient, and his artistic work is mostly made on large format film rather than digital.
Many instead use MFD from personal reasons, they appreciate the quality themselves, but actually don't
need it for satisfying their customers. And then there are different personalities, some want the best because they just like to use the best, and some don't want to spend more money on (any type of) gear than needed and instead get more profit. The same can be seen when it comes to picking a car for professional tasks, some pick the most cost effective alternative that does the job, others pick something extra because they know they will be driving it and like to have something extra. I would not call the more economical person the less professional one... some are just more gear junkies than others
There's also the special feature needs though, if you're used to high sync speed and use it all the time, you probably want to keep that, and that you happen to have 80 megapixels is because you followed the upgrade path for the system, not that you actually need that level of sharpness. Resolution aside, there are still aspects of the Hassy and 645DF systems that this Pentax can't do.