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.
No, what actually makes sense is a proper unification of different copy/paste buffers that is nowadays still mostly improvised and only achieved through very different 3rd party tools (for me using the panel from xfce it’s xfce4-clipman for example that keeps highlighting text and middle-click buffers synchronised with ctrl-c/ctrl-v or ctrl-insert/shift-insert…).
The problem is not accidently pasting something with a middle-click, but not knowing what is in one buffer, what is in another one and which one a program is using.