Like with the CA 99 corridor, any push for Interstate status for CA 58 will have to come from political forces in the affected regions; there is little possibility that Caltrans itself would initiate such an action. Local political action was responsible for the last three Interstate conversions (110 in '81, 710 in '84, and 880 in '86); in each case, a congressman from the area (the late Glenn Anderson, D-Gardena, in the case of the first two) was on point for the necessary AASHTO/FHWA vetting. Part of the problem is that the actual construction involved in Interstate upgrades is considered within Caltrans to be just another drain on their resources -- and that the budgeting process that is involved in apportioning funds to each district would be substantially disrupted by "piling on" such an outlay in addition to the requirements of the STIP under which they are operating. Unless an improvement to Interstate standards on one of the corridors in question is actually contained within the currently effective STIP, even considering 5 to 15-year windows for upgrades to any is thought of as an "off-book" project. It's not like it was with the pre-'73 Division of Highways; Caltrans is an omnibus tranportation/transit agency with as much $$ volume going to localized (mostly urban/suburban) programmed projects -- just examine any of the last three or four STIP's on a line-item level! Long-distance corridor work, unless severely substandard facilities are involved (like 99 between 198 and the Fresno area), is undertaken if and only if there is sufficient funding available -- which, given the more recent bias toward street/transit issues in denser locales, is not always a given.
In short, Interstate conversion isn't cheap -- and Caltrans' priorities are presently focused on localized issues; they won't run point on such long-distance upgrades. If either CA 58 or 99 is to gain Interstate status -- despite being warranted by commercial traffic stats -- it will come from outside. Since the 99 corridor is already listed (via an addendum to the HPC 54 description) as a future Interstate, the only legislative action required for signage would be for a member of Congress to pick a number and tack it onto the existing legislative description; the existence of the state plan to expand 99 would likely serve as a de facto "25-year-window" for facility compliance. Arranging for the same treatment for 58 would require an entirely new corridor designation (Barstow to Buttonwillow); the process would be similar if not identical to the language added to the yearly federal funding bill that eventually resulted in I-42 in NC. Again, that would have to come from a congressional source with sufficient clout to steer it through the committee process. In the case of either corridor, it's likely that Caltrans would approach either "mandate from above" with their usual shrug of the shoulders; they'd likely spread any upgrade projects over as many STIP terms as they could, and carry on as before, with work progressing at a marginally faster rate than before.