[Rover]

Yeah, but then 8mm would have been very extreme on APS-C.

From a business perspective, 10mm is a sound conclusion whereas f/2.8 has more something to do with reputation rather than real appeal. Seems as if the message, that mirrorless systems are optimized for size, has not been received yet - but then they'll also offer this for DSLRs.

I thought it's the other way around - it's a DSLR lens, and the mirrorless versions are an afterthought (since they're just the same, only with longer barrels to compensate for the difference in flange distance).