"If California stopped growing alfalfa...we could probably house 20-30M more people". I'm very happy to stop growing crops in the desert, and agree there's absurd waste there (just because something can be done does not mean it should be done). But 10s of millions more people...is that really a goal, or even a good thing?
The grid can always be greened fairly quickly. That's an easy change to make, and happening quickly in places like Texas where legacy expensive fossil fuel generators aren't protected by utilities. Now that renewables and storage are cheap, any new natural gas plant will soon be a stranded asset, and generators on ERCOT are quickly realizing that.
A much harder change is transportation. We can't build enough electric cars fast enough for the transition, and land use patterns in Texas typically require cars even more intensively than California. In contrast, biking to work in the Bay Area is pleasant newer year round, and there's not much mass transit but at least a little.
The Texas lifestyle requires a car, and people that are forced out of California typically leave in the blue low-carbon areas on this Tablueau map, but will likely move to the red high-carbon areas.
https://coolclimate.berkeley.edu/maps
At least, that's the anecdata I have from people I know, but it's not proper data.