OK, sure, the tech will advance faster than in our timeline, I didn't even dispute that. If they work with Cali, I've no doubt they'll surpass 2-years-per-year. However, a US that's cooperating with Cali, that's receiving and accepting technological aid in the form of advisors and industry, isn't one that's in a tremendously adversarial relationship with Cali, who will hardly be sending tech and materials to a country that is actively raiding them and their allies, after all. When China and Meiji Japan adapted, they had technological advisors and industry brought in from the west, and while the deals were hardly fair and balanced, or treated the Chinese or Japanese as equal partners, fundamentally, if France, say, stopped working with Japan, England and Germany might still be willing to help. The only source of this tech aid is California. Sure, they can't prevent science books and all tech examples from getting through, but to get from before the popularization of true interchangeable parts all the way to an early computer economy in 50 years, without any industrial aid and active trade limitations (once again, positing a hostile relationship with Cali, a more friendly one makes it probable) seems unlikely to me.
Technology and knowledge are fungible. Especially in a capitalist system. Sure, for the first 10-20 years those nations who have friendly relations with California will have a huge advantage in modernising, because they can simply buy brand new production facilities and tools. For the next 20-30 years the nations with friendly relations with California will still have a big edge, because they will be getting direct contact with the leading technological hub.
However, unless California turns into a hermit kingdom and/or manages to pull off the sort of technological control that the west have never managed to do with any other country (including the USSR) then it is flat out impossible that the USA will not have caught up fully in 50 years. Especially when you consider that a lot of people in California will have lingering sympathy for the USA and that slavery will be resolved within a decade.
And note I am saying flat out impossible. Technology does not increase in "2 years per 1" or even "4 year per 1". That is not how it works. You flat out skip decades, skip technologies, and again in a free market economy these technologies "want" to go to where there are natural resources and people to use them. And the capitalist class will be screaming bloody murder if you try to stop them.
Are they really, though? There was legitimately a Popular Union fusion candidate that had the downtimer as the presidential candidate, which at least seems to me like the California Labor Party is willing to treat the US, if not as an equal, at least as being worthy of negotiation. If they've been treating the Slave Power, the south and her candidates, as a "shithole country (or region, I suppose)" as it were, then boo hoo, slavers can go pound sand, but it isn't the North, the part that at least pretends to care about people's rights, they're attacking, "stealing" slaves from, or bombing the military arsenals of. Will there be conflicts at the border, for some time at least? Surely. The US did just shoot down the passenger plane, and put their embassy/whatever and airport under military siege. One way or another, that needs some conclusion, whether that be an actual war, President Cass's impeachment, or whatever else.
I bolded exhibit A for treating the USA as a shithole country. They are basically claiming extra-territoriality for an embassy (with airport) that the hosting nation would like to expell. Then they are continuing to fly airplanes out of an active warzone, and crying crocodile tears and pretending that the USA is oh so mean for shooting down an airplane.
And sure slavers can go pound sand, but you are still planning on destabilizing a large region of the USA and (if need be) fomenting an uprising there. Again you seem to think that being on the right side, that slavery being bad, etc, means that you are not basically acting towards them like the west acted towards any former colonial nation who had institutions the west disliked.
Then there is the way that California acted unilaterally to use Active Denial Systems against that column of troops that were simply moving towards said embassy and airfield. That is you intercepted and stopped a military unit moving around in its own country, while supposedly in a state of peace.
Now... you can make a moral argument for why slavery is so bad that all of this is justified. In fact. Let us grant you are morally right. This is granted. Slavery is that bad. But California are still picking a fight, over and over, and still behaving towards the USA as the west would towards a third world nation that is misbehaving.
I mean I think the
Rambouillet Agreement is a good example. Yes, Serbia was behaving very badly (to put it mildly), and yes an intervention was justified, but the terms of that agreement was the sort of humiliation that no nation with a shred of dignity could accept. I think a lot of the problem with that agreement was that the west was so obsessed with the moral aspect that they could not "see" Serbia and Serbias interests as fully legitimate (or "see" the inevitable effect of enforcing it).
EDIT: Oh and yes. I will accept that settlers won't be the major problem of California in the west, though I still think the border will need active operations that goes for all borders. It will still be a constant issue though.
EDIT2: But I forgot. Even though those territories are mostly empty, they are still, all, areas that were recognised internationally as US territory. So California did not just seceede, and note you could make an argument for why this might still be legal (e.g. the issue of whether states can seceede or not was not yet settled). However, they grabbed a bunch of US territories and there they really have no legal argument. A moral argument perhaps, but no legal argument.
I mean like daaaaaaang... This is a flat out, all out land grab gussied up in humanitarian rhetoric. Not only that, but one done explicitly to deny the USA their "Manifest Destiny."
That is not even mentioning that there'd be thousands of US citizens already living there. Sure the settlement is really, really sparse, but its not non-existent. People who, incidentally, were not asked about the Californian annexation... So are the people that are already there allowed to stay? Allowed to bring their families in? Allowed to expand their farms?
I was not kidding when I said that Golden Island... was basically showing a lot of the ugliest traits of the ugly American.