Let's see how quickly this gets shot down.
So the CZ would go down from EMY to make stops in Oakland Jack London Square, San Jose, Salinas, San Luis Obispo, Santa Barbara, Los Angeles. Using the CS schedule,
5: EMY 5:20pm, Oakland 5:35/5:50pm, San Jose 6:55/7:07pm, Salinas 8:48pm, San Luis Obispo 12:20am, Santa Barbara 3:02am, LAX 6:00am
6: LAX 8:10pm, Santa Barbara 10:40pm, San Luis Obispo 1:35am, Salinas 4:28am, San Jose 6:11/6:23am, Oakland 7:24/7:39am, EMY 7:54am/8:04am
Sure, there already is the SWC to go between CHI-LAX. On the other hand, some people at this group have taken the TE between CHI-LAX and there is a thread about wanting to go between CHI-SAN via the CZ so if the CZ goes to LAX then that would be made much easier as it would be just one transfer in LAX and the times would be very easy. In addition, the extension would give one seat rides between Chicago-San Jose, Chicago-Oakland, Chicago-Santa Barbara and also Denver-Los Angeles, Salt Lake City-Los Angeles, and Reno-Los Angeles.which don't exist now as well as overnight service between the Bay Area and the Los Angeles area.
Running the CZ an additional 469 miles between EMY and LAX seems expensive. On the other hand, if you do that, you can close the long distance facilities in the Bay Area. How much does it cost to maintain those facilities vs. running the train those extra miles? Hopefully the ability to close the facilities in the Bay Area helps pay the cost of the additional service (and eventually they will make back the money with the additional revenue).
As for getting the OK from the host railroads, Amtrak runs tons of trains between LAX and San Luis Obispo and SJC and EMY right now so one more can't be that big a deal. The only one that could possibly be a deal breaker would be the 203 miles between SJC and San Luis Obispo.
Sure you can say once CASHR is coming this service will be obsolete. And how many years away will that be? 10? 20?