Er, no. It is inevitable, but the degree of change absolutely continues to matter.
> US government does not even believe that any such thing is happening and has relaxed EPA regulations on polluters and such, so is CA restricting sale of fossil fuel vehicles going to matter in the long run?
Here's the context you might be missing: CA is a huge automobile market, and has historically set a number of requirements that automakers eventually incorporate into all cars sold into the US market. It's cheaper to build one SKU than two, as long as the CA SKU is only mildly more expensive than the non-CA would be.
Additionally, auto manufacturers will have similar requirements for the EU market on a similar timeline.
Operative word being 'the long run'. The current US government may be replaced within the next few months with one that might be expected to take a more normal view on the problem, so what it's doing now is arguably fairly irrelevant, when you're talking about 2035.
Many European countries have recently made similar moves, with the EU as a whole and China making noises about something similar (though probably on a longer timescale; 2040 or later). So this isn't just a weird California thing.
Just because the USA has federal administration that doesn't want to act on climate change right now doesn't mean that it won't get a greener administration in the future.
As for being a lost cause, isn't it preferable to minimize the effect, at least? would you rather the sea levels go up by 2.0 meters or 1.5 meters?