I think in California with the devaluing of the US shields, any that still are in existence are considered to be 'major routes'. US95, US97, US50, US395, US101, etc. So, bringing back US 6 and routing it down to LA via CA14 would make it another 'major route'.
Plus, the one thing I've noticed talking to people is that changing route numbers, etc in directions is a little much for the road-weary. Even if it's the same road. That's what made PA60-US22/US30-I279 to where I lived so confusing. It's the SAME ROAD I would tell them when they started getting confused. Do not exit, it's the same road. Sure enough, somebody would get off in Robinson on PA60 toward the east, or inadvertently take US22/US30 to the west. That works the same with long distances. People like to stick to one route number for as long as possible. I-70 to I-370 to I-270 to I-70 through St. Louis will confuse a) those without maps, b) those without a sense of direction and c) those without patience. They'd invariably stay on I-70 through downtown simply so it would be less confusing for them.
Signing US6 to LA, again, could make that a preferred alternate to the crowded interstates from SLC, bypassing Las Vegas and going more scenic. Who knows. But, there's no sense truncating a transcontinental route when it's not superseded by something higher (i.e., interstate designation).