Found this article to be useful regarding interstates west of I-5

[roadman65]

This article I find interesting as it brings up points of why Interstate 5 will always be the westernmost interstate running north and south. 
https://www.kcet.org/shows/lost-la/why-isnt-there-an-interstate-1

I used to think that it was because the demand is not there and with I-5 being very close to the Pacific Coast it would be redundant to have another one so close to it.  However, I was surprised to find that the topography of the land would also make building and interstate quality freeway almost impossible as the above article states.

[sparker]

This article I find interesting as it brings up points of why Interstate 5 will always be the westernmost interstate running north and south. 
https://www.kcet.org/shows/lost-la/why-isnt-there-an-interstate-1

I used to think that it was because the demand is not there and with I-5 being very close to the Pacific Coast it would be redundant to have another one so close to it.  However, I was surprised to find that the topography of the land would also make building and interstate quality freeway almost impossible as the above article states.

Obviously the author wasn't familiar with the tentative/speculative 48.3K system that Adam's research brought to our collective attention; it included an Interstate along US 101 between L.A. and the Bay Area along with the original I-5 alignment up (then) US 99.  Obviously, placing a route in the CA coastal region was at least considered at one point prior to the Eisenhower-era retrenchment to the approximately 40K network.  Of course, this was well before the California Coastal Commission was even a zygote -- so such a route would be politically & fiscally problematic today (particularly in the coastal area above Santa Barbara). 

[Henry]

I've seen proposals for Interstate 3 as well. In a perfect world, I-5 would go through Bakersfield and Fresno, while I-3 would completely bypass them, but we'll never know that now. Makes better sense to have it here than all the way on the other coast (GA, I'm looking at you!).

[paulthemapguy]

I'd like to see an Interstate connecting San Jose more directly with points southeast.  I think that would actually be highly useful--the 580 isn't as direct a route as it could be for points south of Hayward.  I don't see why it would be so impossible or awful to get US101 to Interstate standard between San Jose and Los Angeles.  I understand the difficult terrain is a constraint, but nothing is impossible--it's just more expensive.

[sparker]

I'd like to see an Interstate connecting San Jose more directly with points southeast.  I think that would actually be highly useful--the 580 isn't as direct a route as it could be for points south of Hayward.  I don't see why it would be so impossible or awful to get US101 to Interstate standard between San Jose and Los Angeles.  I understand the difficult terrain is a constraint, but nothing is impossible--it's just more expensive.

More than one person has speculated on an Interstate-grade expansion of CA 152 as a San Jose outlet to the southeast -- but the fact is that between lack of funding and disagreements about alignment even an extension of the current expressway that ends at the CA 156 junction isn't a sure thing.  The issue with US 101 is, as I said in my previous reply above, the interface with the Coastal Commission as well as the difficulty and expense of deploying a freeway along the Cuesta grade north of San Luis Obispo.  At this point in time, an Interstate upgrade of US 101 isn't likely; whether a cutoff over to I-5 could be built is purely speculative (and should be reserved for Fictional).