I wouldn't say that a road that could be used as an alternative route to bypass traffic through a bottle neck as an alt route for commuters would be best tagged as a trunk road, especially since I 15 is the main route through here, effectively making it (if we were to be technical and go by the literal definition of a "trunk road") the trunk road through this area. Major alt routes seem like this one example seem like a good case for primary. Not to mention, this road isn't an expressway in the present day so that it can handle commuter traffic, rather this is an old alignment of US Route 395 that has been repurposed, now effectively serving as a business route/frontage road. It's pretty clear to me judging by the addition of bike lanes as well that the intention is to just reintegrate this old highway segment into the highway system without going out of the way and demolishing the interchanges/overpasses. Another attribute is that the traffic volume of Kearny Villa Rd is way lower than I-15's. The reason I-15 gets so backed up and forces some amount of traffic onto an arterial road like Kearny Villa is because it is the primary through route handling the vast majority of long-distance traffic.

Additionally, I wouldn't tag all expressways as trunk, especially short, disconnected ones and/or ones that are already bypassed by motorways. Expressway-based trunk classification is something that the OSM US highway community has been trying to eliminate for a while as physical construction does not necessarily equate a road's importance (while trunk roads are intended to be the most important of important roads, which may include expressways, but not all), and I think the California highway mappers have done a good job trying to get rid of these "trunk stubs".