As long as people are interested in eating, drinking, partying, culture, socializing and having casual sex - cities as most desirable places to live are inevitable.
The gentrification could be viewed as a very subtle form of segregation - that you could just price out people you don't want to mix with from the places you deem desirable.
Eating, drinking, partying and casual sex are just that - culture. Culture is subject to change.
And this isn't what drives urbanization. More simple optimization with regards to commute time and employment opportunities dominate this preference, except maybe in very select areas with high density of youth with enough money to have post-materialistic consumption behaviours.
Diversity and plentifulness of nutrition, social/sexual opportunity, and economic opportunity are strong attractions of cities rooted in our core biological needs.
That more general form combines the ideas that the two of you present (I think efficiency is significantly enough intertwined with economic opportunity that it makes sense to treat them as a single factor). Different people may emphasize differing core attractions as reason for wanting to be in a city, but I think all of these factors are core to what attracts significantly large groups of people to cities, and I think you lose a large part of the attractions of cities by leaving out any of these factors.
edit: oops, originally cited the wrong commentor, also fixed a typo
> all of these factors are core to what attracts significantly large groups of people to cities
And these things can change. Or more precisely, technological or social shifts may change these preferences drastically, which is why real estate and land ownership in general has a black-swan style long-tail risk, which is seldom mentioned.
Primary draws of the city may become weaker with better technology. And we might find [more striking and concrete evidence for] significant drawbacks, especially in term of health effects.