Surprising that this thread has been up three days and we haven't seen FritzOwl's "My plans make CA-138, CA-18 and CA-62 coast-to-coast Interstates and number them I-12, I-12.2 and I-12.4"
Re: first new freeway in L.A. County: in addition to being an extension rather than a whole new route, the 210 extension also had only about 5 of its 26 miles in L.A. County, so I'm not surprised it doesn't really "count."
As far as the route itself: how much traffic is currently going between Lancaster/Palmdale and Hesperia/Victorville? It seems to me like those are mainly exurbs with people either working within each urban area, or commuting into L.A. and/or the Inland Empire, in which case this route doesn't really help anything.
"Twinning" the entirety of CA 138 as expressway (procuring the ROW for freeway upgrade should that ever be necessary (which I doubt)) between I-15 and CA-14, and between CA-14 and I-15 at Cajon, and CA-18 between Llano and Victorville would seem to me the easiest solution. There would need to be new alignment in Victorville and in Palmdale and perhaps a little bypass here and there, but a whole new alignment doesn't seem like the best use of the money vs. need.
As far as long-distance traffic - if the goal is to get from Palm Springs and points east to Bakersfield and points north, wouldn't a better plan be continuing to upgrade CA-58 and then looking at CA-247 and CA-62 from Barstow to I-10? OR, from Phoenix and beyond, just using proposed I-11 to I-40 to CA-58? If the Victor and Antelope Valleys continue to grow as they have been, then long-distance traffic is just going to get mired there, instead of in L.A. Maybe not quite as bad, but I can easily see both those areas topping a million people within 30 years, so it would still be messy.