[mhd]

In the beginning, there was also a good deal of people coming from NeXt and finally finding a target audience. Some of those applications fared even better, as they were basically Cocoa already, whereas some of the older Mac apps were more baroque, not supporting all the nice new features and having a more outdated look. So the "newcomers" served as important style guides. (Let's not even mention the backlash against further outliers like Java Swing apps or RealBasic)

For a while, the OS X app space was rather "pure". A minimalistic aesthethic, plenty of small opnionated productivity applications. Apple itself didn't wreck or disrupt it too much (looking at you, MS Office).

Then mobile happened.

[zapzupnz]

That's a great point. A lot of the best macOS software came from, and still come from, developers from the NeXTSTEP era. Some have faded a bit, but they're still there setting the benchmark.