The main problem with introducing a new short registration mirrorless
system is that other things would have to make room for it. Yes, I'm talking about the long-awaited FF K-mount product line.
If they did that, they would lose those of us who were waiting (even if only recently) for it. They would lose APS-C users wanting something (perceived as) better. But, that's not everything; people will lose faith in the future of the K-mount. It would be a reasonable conclusion, seeing that:
a. most efforts would have to be put into the new, incompatible* system
b. there will be a K-mount product now and then - e.g. they announced only one APS-C lens this year. Yet the K-mount was neglected under Hoya - there's so much lost ground to recover.
At this point, such a move means putting all your eggs into a risky, mirrorless basket: K-mount would be hurt, 645 line can't exist independently, Q is insignificant. If the mirrorless would fail - and we can see MILC makers struggling, years after being on that market - bye-bye, Pentax/Ricoh.
This is a "all cameras must be MILC,
at any cost for the companies involved" strategy. It doesn't work that way.
OTOH what they're doing with the K-mount FF product line is strengthening the entire system. New K-mount FF lenses? Many/most (all?) useful on APS-C as well. Easy upgrade path, and you can start with the lenses. Confidence in a company which is investing on Pentax.
And later down the road, with a stronger K-mount, there might be possible to add a fourth mount without hurting the existing ones.
* I'm talking about forward incompatibility; backward compatibility is supporting the new system so it won't help at all people wanting to stay with APS-C/DSLRs.
mecrox, how many would want to use the D FA 150-450 on a tiny Nex or m4/3, with no autofocus and no stabilization?