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.
I only use a few applications where middle click panning makes sense, and it works fine, middle click paste has no effect. If some applications don’t handle panning properly, that’s a bug in those applications. Why on earth should we disable a useful feature system-wide so a couple buggy applications can get away with shoddy code?