The behavior for browsers without support is to show the google.com/amp URL as before, along with a small html-based bar with additional information about the original domain and share intents.
Does that mean that the Google+ button is coming back? Seriously? Why not just serve the content and leave it at that? Is the tiny bit of extra data you get from a unique "share on Facebook" URL worth it?
I didn't know the Web Share API existed, but based on the RFC, it looks like yet another Google-driven "standard." I still don't see why it needs to be added to the page.