I know several people who use or used the Pentax professionally. The ones who stopped using the Pentax did so due to buffer and tethering issues, went back to Canon or Nikon. I would also point out that the DMF market is increasingly made up of enthusiasts, not commercial shooters.
I work for an agency. I'm not an Art Buyer, AD or CD, but I work closely with them, and here is what I see: I look at lots of files from lots of systems, mainly when they get back from the retoucher and copy is added. Proofs. I can't tell the difference between any of them, for the most part. Slight differences, sure, mainly due to a larger sensor or smaller sensor, or 2:3 versus 4:3.
Here is the dirty secret, from an agency point of view: so long as it tethers well no one cares what camera is being used. No one cares. At all. The IQ from any FF DSLR up to IQ280 is almost indistinguishable for (most) advertising. Beauty is a notable exception, some very high end fashion as well. What we look for is a look that matches the AD or CD's idea. It can be general, with a specific shooter in mind, or it can be just a style. We only hire people we know can execute whatever look we are after, usually represented by a mood board. No one cares how they do it. No one looks at a DMF camera and says: "Look at that camera! This guy must be a pro." If we are paying $80,000 in fees and usage the shooter better be a pro, and believe me no one without significant experience and recommendations is EVER hired. We trust who we hire to do their job as an independent contractor. How s/he does it doesn't concern us. What does concern us is that the camera tether. Period.
I like MF cameras. I really like Leaf Aptus 75s files. I regret selling mine. I've handled the Pentax. I like it. I think it is a better camera than the Mamiya AFD3. I think it is as good as the H, but you get a FP shutter so you can use whatever lens you want with an adapter.
I am not aware of many professional photographers using Pentax. Perhaps there are, although I seriously doubt it.
As far as upgrade paths - you sell can get a module (back/body) on the second hand market and get another one also at the second hard market - that is significantly less expensive that doing it at the dealer's. You can always put it into warranty for $1000.
I have an Aptus II 10 that I bought second hand for $12000 and does 56MP. Sold the old one for $6500.
I can upgrade it at the dealer for about $14K to a new Credo 80 - top of the line.
I would say those are pretty good upgrade options, and doesn't seem to me 4 or 5 times more expensive than Pentax.