Same with Interstate 40 west of Barstow along CA 58. I just wish CA 210 and CA 905 would become Interstate 210 and Interstate 905 at some point within my lifetime. Ditto for CA 15 becoming Interstate 15.
I-40 west of Barstow was an actual submission for chargeable funds, but that was during 1968. The Interstate corridor on CA 99 was never formally proposed, CA 210 was withdrawn from consideration during the late 1990s and CA 905 has never been formally considered.
Regarding CA 99, the simplest answer for an non-chargeable Interstate corridor is either I-305 or (even better IMO) I-99. Neither of those have the legislative number duplication nonsense to deal with. I-99 won’t make many road people happy (it would make me happy) but it would keep the corridor the number it always been. Maintaining the 99 number would probably be favorable to local interests. It certainly isn’t less strange than I-238 and still arguably a better use of the number than east coast I-99.
I rather see a return of US 99 (+ upgrading the portion south of Sacramento to Interstate standards) TBH
I've said before that California should have done what Oregon does with their US highways, and turn them into de facto business routes/spurs. All the various CA-2xx highways could have been kept as a unified US-99 (273, 265, 263), and thus deviate from I-5 when necessary. (Just like how US-30 works with I-84). Had this been the case, there probably would have been enough justification to keep US-99 around in Oregon and Washington, too. After the '64 renumbering, numerous state highways existed in pieces anyway, so US-99 wouldn't have been any different in this regard.
But that's not what happened, obviously.