Gary Illyes responded that Google is looking to see a "quality neutral" launch. That means that when they flip the switch to be a mobile-first index, they don't want the search results to change much. They want the quality of the search results to pretty much be at the same level or a bit better. If they do not see a "quality neutral" data set, they will not launch.
Getting to "quality neutral" is not easy and it is tricky he said. They are looking at a whole new web, a mobile web, and it is a whole new index. So it is tricky.
Gary has said this before but always nice to hear him say it at a show.
Here are some tweets from the session on this point:
Forum discussion at Twitter.