First, I know they aren't eliminating the feature for current users, at least not at this time. And I know I can share any file in any folder. Of course this is deliberately misleading, because the sharing isn't the same. We're all smart, so I don't have to explain how. And sharing is the one feature that makes a difference if other people have it or don't have it.
So why? I don't know. But I can take an educated guess. 1. A small number of people were using the feature to store graphics or audio or video files that were getting a ton of traffic. 2. They're getting a lot of takedown notices as people use the feature to share movies and music owned by the entertainment industry. Net-net, the feature is expensive both in terms of bandwidth and human cost.
I'd bet their plan is to limit the damage by not giving the feature to new users, and then weed out the abusers, to try to contain the problem. If they can't, they'll just shut down the feature for everyone. They'll say they're sorry, but it can't be avoided, and you can use the new sharing feature, that doesn't have these problems (for them).
It sucks, because it was a uniquely valuable feature for people who were just using it to store documents. It was especially useful for outlines containing feeds, templates, stylesheets, reading lists, data to be consumed by our apps. The user can specify a URL of a feed and we can read the feed. Updating the data, for the user, is as simple as opening the file and saving it. Now, they'll say we can use their API. But that's more work, and I probably won't do it, because that's the beginning of a long trail of changes, deprecations, and new limits, and strategy taxes. Your work is never done when building on corporate APIs. So I don't go that way.
I put this in a thread so people could add new information if it's available, or ask questions.