I think the fact that 1600p monitors haven't dropped in price in the past few years despite all the super high resolution phone and laptop lcds is good evidence that the price wont be impacted by 39"+ 4k screens. They will either be much more expensive than 1600p or the sizes wont come down to a manageable 30"-32". As a gamer I run into a lot of issues even at 1600p where UI is too small to read and there are no options to change it. The 30" size also increases the amount of saccades and head moving to read parts of the UI or see points of interest. Even when it works well, 30" is about the max I'd want for some games, and I don't think a higher pixel count at 30" would make much difference unless you had way better than 20/20 vision. IMO, where 4k would really shine is in the larger screens (aka TVs) more than computer monitors.
As for the availability, I don't see how anyone could know that unless they are a higher up in one of the companies and they likely wouldn't be at liberty to say. I'd like for prices to come down too though, but I think I'd probably buy another 30" 1600p even if the slightly larger 4k monitors were similarly priced.