The 31 and 77 are special from design but essentially for the rating of the glass element used. Any lens manufacturer can do it at this cost, but they don't, they use lower grade glass elements instead in order to match a certain price target. The DA* from Pentax are kinda the same sort of the Fuji primes, except that Pentax has only the DA*55, and Fuji made a set of primes of this kind. Evan Sigma and Tamron can design fast primes that kinda get you the image clarity of the FA ltd, but those lenses are priced in the range of $800 to $1000, not the $400 you paid for a DA lens or the $350 you paid for a Tamron/Sigma lens.
---------- Post added 22-11-16 at 10:54 ----------
Originally posted by BarryE At the moment I'm struggling to love the DFA 24-70. The move from DA primes and DA* on a pair of K-3s at low ISO to a K-1/24-70 has not been smooth. Yes I have more to work with and greater flexibility with the K-1 but I'm finding that I'm also carrying a K-3 too and often preferring the shots from this. I wish I could decide whether to try the 31/77, albeit with the loss of zoom capability, especially as we're getting towards the holiday present season ...Maybe I could then love my K-1 and move off the K-3 ...
I understand how you feel. Image quality with primes on apsc rivals zoom on FF, the FF zoom combo is larger and heavier, you have have the flexibility of a zoom. But lets wait and hope Pentax release a couple of new primes with the quality of sigma art lenses / tamron high grade primes, you'll see how the full frame + prime combo will outdo your apsc+prime set.