>Another thing, if they weren’t communists, where is the incentive to keep up the charade with all the symbolism? Surely a rational non-communist polity would realize there is a lot to gain from dropping the whole red, socialist realism and adopt a more (post-)modern aesthetic.

Much to the contrary. All governments strive for continuity to legitimise their regime. It's why for example even revolutions often co-opt the institutions they're trying to replace. In this case the communist "branding" is what gives them their legitimacy.