Part of the apprehension might be caused by the difficulty of rezoning itself. The only people with the determination and money to get a zoning variance are big developers who need a big building to make it worthwhile. That's how you get 50- or 100-unit apartments going up in single-family neighborhoods, and a "missing middle" of density and affordability.
Forget that ! When you go to a dense city like Barcelona where every block is nearly 4 story high rise apartments, its wonderful.
Change is beautiful, people enhance neighborhoods
That's a pretty remarkable belief. Many of us happy suburbanites have lived in large dense cities in our younger days. We are not ignorant of the joys of city living, we've just changed priorities.
The extreme example is of course Venice.
I chose a random small city in northern Catalonia called Vielha and viewed it on Goggle Maps, there they are. They look different to Eixanple apartment buildings, but they are there in the centre. Population less than 6k.
Chose another, Gironella, a bit further south; population around 4k. One less floor on the multi family apartment buildings in the old centre, and a much higher ratio of SFDs to MFDs in the settlement as a whole, but still there.
How many should I sample to convince you?
Zoning that mandates low densities is used to economically segregate areas. They're invisibly gated communities enforced by the government. "Can't live here unless you can afford to buy/rent this much land."
One of the things I noticed while living in Munich was that the differences in apparent neighborhood prosperity in different parts of the city were vastly less extreme than in major US cities. Almost every neighborhood looked kind of vaguely middling, it was rare for a place to look obviously well-to-do or obviously run down.
And it makes that it's harder to get extreme differences there if at least medium density dwellings are allowed just about everywhere. Working class people may not be able to afford a whole house, but they may be able to afford an apartment in a given neighborhood.