The language and notation is inconsistent with hundreds of years of use in maths and confusing for kids learning both maths and programming (at least it confused me).

Regarding your skeptism of immutable data structures, it's best to think of immutable data and data structures as copy-on-write instead of update-in-place. This is a technique that is currently very underused, for example most programming languages do not come with "persistent" copy-on-write collections.

Here is an example text editor built using immutable data structures in C++: https://github.com/arximboldi/ewig

Advantages are excellent support for concurrency (background save works while you edit) and trivial support for undo/redo.