I generally would agree that touristy city centers tend to feel similar to each other, be polished and therefore boring. However, your example of Hanoi couldn't be more hilariously misguided. Your characterisation of touristy areas ("places of quaint storefronts mobbed with American retirees, hip bars filled with plastered 25 year old Brits, and a few monuments with millions of Instagram posts. Despite being in very different cities, they all feel the same. You have your five star hotels. Your restaurants that everyone says you have to go to. Your buildings plastered with historic plaques") is completely off the mark. The old town of Hanoi is precisely where locals go in the evenings for a cheap beer on the streets - it doesn't get more typically Vietnamese than that. And the old town of Hanoi definitely does not feel the same as anywhere else, even just within South East Asia, Hanoi is known as a very special place.

Don't get me wrong, exploring other areas of Hanoi is a wonderful experience as well.