I think its pointless to talk about the word legacy as if its a bad thing; one needs also to discuss why the legacy design is lacking in some respect.
The world is full of legacy designs that still work quite fine and reliably. Writing paper is a legacy design and requires no batteries. Graphite pencils are legacy design and more of them are sold than any other writing instrument. The claw hammer is a legacy design and still works quite well, etc.
So what is it about the K-mount that makes it unsuitable for MILC? On a K5, those lenses, with SMC still take fine pictures. I think the 3 negatives are (and i'm guessing pretty much):
Cons for K-mount in MILC
a. They aren't physically compact, and thats what MILC is sort of about.
b. They aren't as suitable for video, e.g. not as silent if screw driven,
c. Don't AF as quickly as some more modern designs (i'm thinking some of the m4/3 which are reported to AF quickly)
However, photozone.de says this about Fuji's compact lenses for the xpro1:
Quote: The AF speed is Okay but, frankly, light-years behind modern SLR or Micro-Four-Thirds lenses. AF object tracking isn't really possible - that's at least based on the X-Pro1. The AF accuracy is very good though. Noise-wise AF operations are noticeable albeit not really disturbing. Manual focusing works "by wire" thus by triggering the AF motor
Pro for legacy K-mount in MILC
a. There is a lot of them out there, and with contrast peaking - can be manual focused quite easily
b. There is seemingly less barrel distortion in the older lens designs, e.g. the new Fuji XPro1 18mm lens has 4.8% barrel distortion and auto corrects it in the camera and in the RAW converter. The Pentax DA 21mm lens has about 2% and that used to be considered a high amount.
c. There is seemingly a lot more field curvature in the modern compact lenses for MILC than the legacy lenses. One would expect the field curvature for a macro lens to be pretty flat. The DA 35mm f2.8 shows a much flatter focus field than the XPro1 60mm macro (the only one they have at this point). (this according to information at photozone.de)
d. Modern compact lens designs from Sony and Fuji, i assume m4/3 are similar, are manual focus by wire, i.e. manual adjustments are sent down a wire to the focus motor. Experienced photographers say that the focusing feel of the legacy designs is superior to the focus by wire designs. (i think its a trade-off in compact and flexibility in design, however. Legacy designs may have a better feel, but focus by wire allows more compact and possibly quieter designs-i think)
Conclusion:
I suspect that Image Quality in the modern compact lens systems is not going to be as good as the legacy lenses as discussed above. With the modern ability to correct barrel distortion via firmware - that issue doesn't matter much to me. However, the resolution in the corners and borders is not as good as the legacy lenses are capable of, and that cannot be restored via firmware. If all one is going to use their images for is to show on facebook or on the WEB, then that loss of resolution hardly matters. The appeal of the Sony Nex adapters, which are widely available, is that one can use legacy lenses on the Nex cameras if one really wants a flat field curvature for top of the line IQ.
I've arrived at a conclusion that the use of legacy lenses in the K01 is not just a convenience to Pentax, but an advantage to the customer in optical quality, at the expense of size. (except for the xs lenses). The K01 is truly in a niche of its own, IMO.