I completely agree with your analysis, and you are right, Pentax 645, and even 645n are still very affordable. Even 67, though in average more expensive, can be still found for a bargain (I had seen locally sell here two for $300 CAD 1-2 years ago, but I prefered going the 645n route).
One point though: the Pentax 645Z is discontinued and "affordable" digital medium format went from Pentax to Fuji (and with this from DSLR to mirrorless). Wouldn't Pentax like to keep that niche it had alive?
When the Pentax 645Z was introduced in 2014, it offered probably the best price-performance ratio among all digital medium-format options. And it has been quite successful in the market in the first years.
Why Pentax has not further continued in that direction? No one really knows exactly (Pentax has never commented on that topic), but my assessment is
- problems with sensor supply (new 100 MP sensors) at reasonable costs
- the success of Fujifilm's offerings in that class surprised them / they were unprepared
- the smaller dimensions of the Fujifilm and Hasselblad mirrorless models have been very positively recognised by the customers / the market (Hasselblad also has meanwhile discontinued their H DSLR system, and Phase One has moved a bit away from classic photography applications to aerial and archiving photo applications).
Wouldn't be a model which could do both, film and digital (if that's possible), have a good advantage over Fuji?
Let's go 5-6 years into the future: Imagine Pentax might internally discussing entering the medium format market again, evaluating the possible options.
If I would be in charge / in responsibility I would definitely suggest to build on the legacy of the popular Pentax 645 system.
But I would avoid / remove the most significant disadvantage of the Pentax 645 line: The lack of interchangeable backs.
A new Pentax 645N-III should have interchangeable backs, because that has always been an advantage and important differentiation factor to almost all 35mm SLR camera systems (with the exception of the Rolleiflex SL 2000F, 3001 and 3003).
The competition in the MF market has also offered it.
And with that feature a potential 645N-III could also offer the option for a digital back as well.
But maybe the target groups for film vs. digital medium format are not enough overlapping?
The number of photographers who are using both mediums parallel will probably be lower than the number of those using either film or digital.
But that isn't the decisive factor: With interchangeable backs Pentax would generally increase the customer basis: Three customer groups (film, digital, both) instead of only one (digital today; film in the past; maybe film in the future).
Best regards,
Henning