The Sigma 18-35/1.8 is a good choice for those who like fast zooms no matter the cost in monetary and encumbrance.
Also not considering bokeh, flare resistance, need for heavier tripod setup for stability.
There is no free lunch here to me.
With a selection of primes, they are smaller and I can mix and match for my needs.
DA15; DA21; DA35; DA50 for example, works out to be the size of one 18-35/1.8 and the lenses form a modular system than stores in various spaces in a bag.
Not take up one big area, leaving little space for other stuff.
Some trips, I can leave lenses at home (or in the safe of the hotel);
say a trip to the local market for street shots and I only need a 35mm.
Or a simple trip down to the local watering hole during a tour where I'd not want to be w/o a camera (so cam+35mm would do fine).
Furthermore, how often is a 18mm needed at f1.8 or even f2.8 for good IQ. (in general, of course there are niche applications like caving, but hope the lens does not get stuck in a crevice
)
For Shallow DOF? - given the DOF of the focal length, its paying a lot for not much.
I think the second part of the OP, was that perhaps Pentax can make faster/smaller/cheaper primes for aps-c.
Generally, we would have to choose between 2 of the 3
(fast/small <> cheap; cheap/small <>not fast; etc)
It can be done, but I'm not sure if its going to be a priority if Pentax has a FF in the works.
A Pentax FF immediately leap frogs Pentax above the Fuji offerings (as well as Canon and Nikon FF) being FF (vs Fuji); small FF lenses (FA ltd; FA; DFA is generally smaller vs any other makes ).
If Pentax is not interested in FF development, then they should focus on faster primes and zooms catered for aps-c. (some lenses like 24; faster 35; 85mm )
Not sure about them being cheap though...