why wait? most of the older pentax lenses are equally good. do a self check on what primes or zoom you REALLY need, go out and buy them! Remember the sooner it ends up locked into your camera body, the sooner you can make things happen! If you are always waiting for that "better" lens, than it would be like the PC-syndrome where a person is constanly wating for the next faster CPU to come along and holding back.
But for your 100-400, maybe sigma will have to do...
Also, if you check out how they make lenses on the canon website, you would know its not easy and requires economies of scale to bring it into production. something canon has plenty of but pentax does not. but its a good thing that for the fewer lenses that pentax do have, they are all great performers...