We're talking about the cache, right? The index, or more likely indices, are optimized data structures used to search the cache. I doubt Google could share those without revealing too much about their ranking algorithm.
Letting sites inject into the cache is an interesting idea, but Google will still have to spider periodically to ensure accuracy. Inevitably, a large number of sites will just screw it up, because the internet is mostly made of fail. This would leave Google with only bad options: If they delist all the sites to punish them, they leave a significant hole in their dataset. But if they don't punish them and just silently fix it by spidering, there is no longer any threat to keep the black hat SEOs in check. Either way, it would cause an explosion in support requirements and Google is apparently already terrible at that.
I think the idea was that only Google will crawl your site and update the index, then the rest of the search engines will use the index instead of hitting your site.
Letting sites inject into the cache is an interesting idea, but Google will still have to spider periodically to ensure accuracy. Inevitably, a large number of sites will just screw it up, because the internet is mostly made of fail. This would leave Google with only bad options: If they delist all the sites to punish them, they leave a significant hole in their dataset. But if they don't punish them and just silently fix it by spidering, there is no longer any threat to keep the black hat SEOs in check. Either way, it would cause an explosion in support requirements and Google is apparently already terrible at that.