> The only pre-20thC buildings that remain are the ones that survive.
This has become a cliché talking-point, but it isn't entirely true. Or, at least, it doesn't fully apply here.
The best contrast to "modernism" would be the episode directly preceding it, namely Art Nouveau or Jugendstil.
There are lots of European cities both large and small that were predominantly built in this style, because it coincided with the rapid growth of cities as industrialisation swept (parts of) Europe. While British cities and Paris are somewhat older, everything east of France saw a major growth spurt in this time.
Because construction was rather centralised, a great many houses of rather similar style were built.
My city, Berlin, is a prime example of this. The core city centre is somewhat older, but about 80% of the housing stock were built within 25 years around the turn of the 20th century.
Here's a street view of my house: https://goo.gl/maps/NRhfef2KEdx7yAET8
If you walk around a bit, you will see newer, mostly ugly buildings. But essentially no houses were ever demolished for aesthetic reasons. Every new house owes its existence to World War 2.
Here's a map showing building age on a block-by-block level: https://interaktiv.morgenpost.de/so-alt-wohnt-berlin/ (click on "Karte aktivieren" to the right). If you hover over different blocks, you will note that for the vast majority, houses were built around the same time. The same is true for wider areas, generally becoming younger as one leaves the centre (as expected for a growing city.
For the "survival of the pretty" theory, one would expect more within-block heterogeneity, with one house surviving and it's direct (ugly) neighbour having been demolished and rebuilt.
Go to cities that weren't bombed, such as Budapest, and you will find entire quarters unchanged since they were built about 120 years ago.