This is written like the author has very carefully read all the Wikipedia articles on the electric system. But they mentioned the importance of inertia, so I'll give them +1 for that.
"Many?" I'm not sure three is considered "many," and I seriously doubt the eastern border of the Western Interconnect at Texas/Nebraska/North Dakota is limiting the ability of California to manage its own grid. The famously poor ties between Southern California and Arizona will limit things long before the asynchronous tie another state further east.
I'd watch Hawaii. A 100% renewable mandate on an island is aggressive, and the solutions they come up with will be able to steer everyone else.