This again, is partially true. A lot of ornamental decorative elements in public facades were, essentially, mass produced using molds --- very few were hand-made in the way they are done now.

They would degrade over time, however they could also be repaired or replaced relatively easily.

Your point about survivorship bias is also somewhat more nuanced. The buildings that have survived were often, as you said, made with more expensive materials, and they lasted longer; but there was also a change in tastes over time, in particular against ornament and decorative elements, and a change in the idea that certain buildings should be maintained. In the past, people were just way more accepting of tearing down or replacing buildings. Conservation as a general approach was not as common, nor were people as inclined to see certain buildings in particular as uniquely worthy of conservation. It is those buildings which were deemed "appropriate" for conservation and "historically significant" which have largely shaped our understanding of "traditional" architecture, a somewhat snapshot idea of architecture on the cusp of modernism,