Republicans would be happy to see California leave, as it would ensure their control of the House of Representatives for the foreseeable future; maybe the presidency. They'd probably even agree to a sweet-heart free-trade deal. And I don't think people appreciate how deep the disdain is in many parts of the country for liberal, Democratic politics in general, and California in particular.
New England and some Midwestern states would fight it tooth and nail as it would leave them, culturally and politically, with substantially diminished power.
America was always an uneasy balance between the North's industrial and merchant ethos and the Southern plantation culture that emphasizes fixed social identity. The problem is that as income inequality, immobility, and non-European immigration has grown, the Southern plantation culture is predominating. It's even infected the more progressive Northern culture by emphasizing identity politics.
Race doesn't "exist", but racism exists, and it's easier to conceptualize and identify racism by classifying people according to "race", such that both perpetrators and victims will naturally self-classify. Likewise, as overt prejudice against various groups has moved into the public sphere in a way that permits it to be challenged, self-classification has grown stronger. Identity politics has always existed, but in Northern political culture it was much less consequential and more fluid. Change came fast and there was less time for pathological patterns to set in. In plantation culture social identity is more fundamental to how power is structured, brokered, and perceived; and obsession with avoiding change means pathological patterns are more likely to set in. This is why, despite racism towards blacks being alive and well in the North, Northerners have historically be more accommodating to power sharing; whereas in the Deep South despite blacks nearing 40% in some states they're still grossly disenfranchised politically, not just economically.
This election is further proof that Southern political culture is spreading like a virus. In most of the U.S. identity is used to exclude and antagonize; in California identity is still largely an inclusive concept (i.e. diversity is still largely seen as an empowering phenomenon, even by poor whites, or at worst a neutral phenomenon). I'd rather not wait to see California go the way of much of the rest of the country.
I grew up in the Deep South, and have lived and worked in the Mid-Atlantic, New England, and Chicagoland. I'd be happy to see California secede (with my family along with it), especially if both Oregon and Washington came along for the ride given the strong economic, legal, and political similarities. Though, that's even more unlikely than California seceding alone. My only hesitation would be leaving behind some other Northern states. The South and rural areas would be almost universally ecstatic to see California go, at least in the abstract. (Some rural white Californians notwithstanding.)
I'm also not oblivious to the fact that the current movement, such as it is, is a peculiarly techie phenomenon and a product of a particular tech-focused worldview I do not share despite my career. But I've thought for years that a California secession would make a ton of sense politically and culturally. The first time I came out here over 15 years ago it felt like a different country; it still feels that way.