That "time frozen in '50s American retrofuturistic utopian design" is a fundamental part of Fallout's lore and visual identity. They could totally go outside that, but it'd be considered a ballsy departure and it'd be a huge chance to get it wrong compared to the safety of the known design.
I'd like to see them do it, but I think there's too much for them to do within the framework still before trying something like that. A bit similar to how GTA is trapped in America due to how core to its identity American culture is.