It's relatively easy to generate that amount, especially when you can easily import 20% of your power(very dirty coal power), and also run gas turbines for another 40%, like it is at this very moment
https://app.electricitymaps.com/zone/AU-SAWhen you're talking about changing the US, it couldn't just import 100GW, Canada and Mexico couldn't provide that much, even if they were clean energy. There is just a difference between providing for one small region vs a continent. Also, Uruguay has huge neighbors, they could easily import-export clean energy from them (they are all hydro-centric).
But yeah, I will see the US going to 70% pretty soon, considering where panel prices are. The one big thing missing is the low cost to install on rooftops. In the US is costs at least 3 dollars per watt to install panels, whereas in the Eu or Australia it's under 1 dollar. The panels themselves can be bought for 15-20 *cents* per watt, so there is a lot of room for improvement. Rooftop solar in the US is a big scam at the moment, they are fleecing the consumers. On the other hand, utility size in the US is cheap, so they will cover the gap.