As someone who also follows the mass transit side (being a vanpooler and commuter, and someone who listens to the excellent talking headways podcast), my guess would be that, for urban areas, the priority would be increasing density of transportation (i.e., more people per vehicle, whether that is through ride sharing, shared rides (uber, lyft, etc), transit, etc), as that is a cheaper option than widening of roadways -- what with land acquisition, utility movement, construction, and environmental mitigation costs. For freight corridors, you'll see more emphasis on less polluting, creater capacity, and more autonomous freight vehicle (as that also increases density and throughput).
However, for the more rural areas where increasing ride density is less of an option, there you might see more roadway improvement -- certainly upgrading of level of service, possibly widening, less possible grade separation (freeways) -- but whatever rural upgrades are done will require justification based on the traffic of the road. Caltrans and the CTC has to be careful: even in cases where level of service might not support rural improvements, making visible improvement in those areas is vital politically, as those are the areas most opposed to increased road taxes and most sensitive to the call for secession as they feel urban areas get all the tax benefits.
Translation of the above, at least to me, with respect to this topic: If the traffic and LOS on J59 justify improvements, or there are specific safety areas to improve (such as slight widening and rumble strips) you'll see those done -- even on the county road, even without making it a state route. Unless the level of traffic or safety requires something more, however, you won't see major improvements, and you'll not see it join the state system unless they either have the need to make it a formal expressway, or the county threatens to abandon maintenance and Caltrans sees route adoption as the only way to provide the transportation option.