Middle click paste is a very useful feature for a lot of people, but new linux users are not those people.
I personally switched two years ago, and got several people I know to switch too. Everyone I know who switched (including me) was confused by middle click paste.
It’s a hard to intuitively understand action (took me several months until I understood it took the selection for some reason) that is very easy to trigger accidentally, and that duplicates existing functionality.
The people who like it already know it exists, and could just toggle it on.
Of course, on distros not aimed at beginners, like say, debian, it should remain the default.
Thing is modern clipboard managers like the one on kde server and equivalent purpose.
Oh I agree, but most windows users I know don’t even realize they have a clipboard manager for history pasting. So may not even be looking for that.
But from a UI perspective select text and middle click is just way less key strokes and fluid.