Changing Pattern of Lens Range

I understand not everyone would agree with these points. I am just trying to guess the Pentax reasons from their approach, which has been quite consistent in the "digital era" (except for the DA*55).

It's true that faster than f/2 can produce some interesting effects, but these are gimmicks in the same way adding effects in PP is a gimmick. Some might like them for aesthetic reasons but it is hardly a reflection of true optics.

There is a lot more to good bokeh than a fast lens. I have lenses from Vivitar and Leica, to give two examples, which produce astounding bokeh at f/5.6.

This is hard for me to understand. I have always thought of aperture as one of the main creative levers open to the photographer. An ultra fast lens therefore gives you more leverage, particularly important on a crop sensor where narrow dof is harder to achieve. On my film SLR I sometimes used to set my 50mm to f2 and put the point of focus where I wanted it. To achieve the same on a crop sensor DSLR I would need something like the 30mm f1.4.

And of course it lets you take pictures at a lower ISO than would otherwise have been possible.

You are not wrong, but my point is based on optical design. For every stop you increase light the aberrations go up by a factor of nine. It becomes almost impossible to control these beyond a certain point, which is why very fast (significantly faster than f/2) lenses generally have all sorts of odd halo, coma etc. effects on top of their general softness. (Certainly some photographers like these effects and use them creatively, as I already mentioned.)

On top of this, the smaller sensor makes optimising the light pathway inherently difficult, as the density of photo-sites / pixels increases.

I fully understand that Pentax has decided to limit their lenses to around f/2.8 maximum aperture, though personally f/2 is more to my liking. Beyond this point I think one is fighting a losing battle. I simply don't think most people would pay the money for a highly-optimised digital-ready lens at f/1.4. (How many bought the DA*55?) not to mention that this would sacrifice the compact size that is a Pentax competitive advantage.

If one wants greater DOF control without sacrificing optical quality, the answer is a larger sensor.

(I say this as someone who owns not one but two f/1.2 lenses.)