MIT means it's only open source for as long as Google feels like it should be (and only the parts they want to keep open). Older versions will still be around to fork from, but maintaining a fork of an OS is a pretty large task.

Android is also open source, and is notably very difficult to simply fork and do your own thing and then actually use the thing, unless you happen to be a handset maker.

My OS entirely driven by Google? No thanks, they're making enough of a mess of the web (and Android lately, TBF).