Well if Google is already hosting all the content in their walled garden (see AMP), this would still be able to crawl the content, and even be able to be the exclusive platform that could crawl that content (in theory)
This is probably straying a little off-topic, but AMP seems nothing like a walled garden when compared to Facebook, Instagram, or Apple app store.
AMP is nothing like a walled garden, period. Anyone can host it.
I was a bit young at the time but isn’t this what AOL tried to achieve?
Somewhat: there was definitely a walled-garden with an entire AOL ecosystem, but that wall also had a door you could walk through out into the wild west web of the times.
True, but fortunately the vast majority of content isn't fronted by AMP. Most of my queries don't return AMP pages.