20% time helps bring in the "entrepreneurial" types of engineers that Google seeks to hire and as it starts to disappear, they might see fewer of those hires and more of those engineers leaving. Perhaps it's simply because Google has grown to the size where it can take on really big innovative projects (Google X) instead of the respectively smaller projects that were created under 20% time. They've found that they can't have both without hurting their bottom line, so they've decided to go after the big projects. It's a significant shift, but in the end, it could be good for all of us because many have the resources to go after the small and medium wins, but only a few can go after the really big wins.