Kosovo

This is the grand hope of every person in power.

I don't know. The fiscal consequences of California's expulsion from the US would be steep to both the US mainland and the hypothetically seceded California. The state contributes nearly 15% of all federal tax revenue (1), has the largest agricultural industry of any state (2), and would likely be subject to tariffs, posing a costly impediment to trade. The effect of imposed tariffs will no doubt be massive given that California is solely responsible for 10% of US exports (4). Granted, California is the largest recipient of federal aid (in total the aid is roughly $47 billion dollars), but the tax revenue provided by California is an imposing edifice in comparison to the tiny sand castle of aid given to California (compare $45 billion dollars in aid (3) to $234 billion dollars in taxes paid to Uncle Sam (5). Also, when measuring federal funding per capita, California is actually the 10th lowest funded state, while Virginia, Kentucky, and New Mexico have the most (3).

1.https://www.money-rates.com/research-center/federal-income-taxes-by-state.htm

2.https://www.ers.usda.gov/faqs/

Slavery was bad for the South's economy, sure, but do you think that keeping California in the US is equally harmful? I don't think reconciliation between the rest of the US and California is necessarily impossible either as it was with the South. There's been no equivalent of "Bleeding Kansas" to separate the interests of California and the rest of the US. In fact, while California is a stalwart defender of their own interests, and may violate national law by declaring sanctuary status, other states have done so to. Many have legalized marijuana partially as well, even it infringes the Controlled Substance Act's Scheduling chart (1), and many more have enacted abortion laws that contravene SCOTUS precedent (2). It might behoove the federal government to be more receptive of state interests when forging policy, or allow some leeway in enforcement.

If California were to leave, there would also be culture shifts related to economic shocks. Generally speaking, poverty trends upwards with crime. While not a clear-cut case of correlation, it's not a stretch to suggest that crime could spike as a result of such an economic shock. California would need to forge its own government structure and constitution, be saddled with debt that will likely never be paid back, and lose access to the US army. The US mainland would have to cope with the loss of the agricultural giant, probably experience an influx of migrants from California  (probably many people who want to emigrate to California too), and would appear weaker to state actors. 

America is polarized, no doubt, and states have differing goals. But this is not the most divisive moment in American history, that prize still goes to Reconstruction for obvious reasons. No issue today could possibly hope to be as salient as the slavery issue. Pundits may abjure opposing political sides and lambaste eachother at a disquieting pace, but we've steered clear of assassination, Congressional brawls, and thousands of lynchings. If the Confederates could reconcile their differences with the Union, I don't see why California and the rest of the US can't.

Sources

1. https://www.dea.gov/drug-scheduling