And I doubt we ever will.
There is an entire eco system involved that, as with Nikon evaluating getting into the cinema market, is just so well developed that it wouldn't make sense for other manufacturers to get involved. Its not just the obvious stuff like where are their 400mm 2.8s but everything else that is needed to get a picture from a camera to a picture desk in seconds under any sort of environmental conditions. That goes from everything like bullet proof wireless and ethernet modules through to the basic robustness needed to cope with the elements and the general physical battering they get.
Its not that we don't want the advantages of mirrorless in terms of the weight reduction and cost reduction (particularly the ongoing highway robbery maintenance costs with shutter replacement) but unless it helps perform the overall task better than what we've got then we won't be changing. Ultimately, as RED found out with the RED ONE, its the overall task that really counts when its being used professionally rather than just the imaging module itself, as thats only one piece of the puzzle. If it doesn't fit into an overall eco system it'll be a big ask to get working pros to take a chance on it.
Because of that, when we go mirrorless - and we definitely will - it will be with Nikon and Canon because the rest of the puzzle is already there.
As for stills extraction, leaving aside the quality issue there are numerous reasons why it would need serious work to get it to fit into an editorial workflow. For one thing, we just don't have time to be scrolling through video to do the extraction. If a goal goes in then we're currently dealing with probably 10 or 12 frames to pick the best one and it then goes in one button press to be ingested, edited, captioned and ftp'd to a picture desk. And all of this is going on while the game is continuing and you're still shooting. Switching modes, scrolling through, marking frames, doing the translation to a still before we even start the rest of it? We just don't have the time for that and I know this for a fact because I've done live trials of it.
However, the biggest issue is a bit more fundamental than that and its to do with media rights.
Stills shooters are subject to heavy restriction at events as broadcasters have paid eye wateringly large sums for the motion rights. These restrictions determine how many sequential shots of a particular incident you can wire and a restriction on the overall number of shots per event so they cannot be reconstituted into a motion sequence. The restrictions are also related to the capture device itself so you can't use a DSLR in video mode unless you hold the broadcast rights. If you are seen using one in this way then you won't only be asked to stop, you'll have your accreditation revoked and removed from the stadium or event.
Because of that, I'm going to say that the amount of pro sports shooters Nikon and Canon are losing D5 and 1DXii sales from to the likes of GH4 or anything else with stills extraction is likely to be zero.