It is my long-held view that States have a constitutional right to secede from the union without the approval of other States or the central government. (See this post, for example.) If the Yes California movement succeeds, the political benefits to the rest of the United States (or at least the conservative parts of it) will be substantial; for example:
The last presidential election in which the GOP candidate won California’s electoral votes was in 1988. There wouldn’t have been a Bush-Gore controversy in 2000 with California out of the picture. And in 2016, Hillary would have lost the nationwide popular-vote tally by 1.4 million, thus putting to rest another baseless claim that the Democrat candidate was “robbed”.
The GOP would hold a bigger majority in the Senate (4 seats instead of 2) and House (74 seats instead of 47), thus enabling Republicans to move national policy to the right with less interference from RINOs.
Illegal immigrants will flock in greater numbers to welcoming California, thus reducing tax burdens and crime rates in Arizona, New Mexico, Texas, and the many States farther north that also absorb illegal immigrants.
According to the Yes California campaign, federal receipts from California are about equal to (perhaps a bit higher than) federal spending in California. Even a slight deficit would be worth it. That could easily be covered by spending cuts that might not otherwise occur because of the California Democrats in Congress.
And even more importantly, as commenter Timoid says, California’s wacky environmentalists wouldn’t be setting policy for the rest of the nation.
Last but best, Nancy Pelosi would no longer be a Congress-critter.