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.
My mom kept accidently pressing the mouse button, I could never figure it out why, when I was watching I guess shed be conscious and use it properly, but complain later that things get pasted randomly. After I disabled middle click, she hasn’t complained.