If by usability you mean ease of commuting then many old cities have been able to address this by building new infrastructure while preserving their character.

If by usability you mean ease of commuting then many old cities have been able to address this by building new infrastructure while preserving their character. Amsterdam is an old, concentric, beautiful and imo quite user friendly city (as long as you don't drive a car). Commuting is very easy either by bike, tram, metro, or train. For me the ideal city is both aesthetically pleasing and easy to get around (among many other things I'm omitting here). Make it beautiful and make it efficient. That's a challenge - but feasible.