This has already been proposed in P1046: Automatically Generate More
Operators, although with explicit = default. It didn't go further than
allowing defaulting ->.
https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1046r2.html
I like the idea, but a strong reason against it is that you can also
default operator++(int) in terms of operator+, with no prefix
increment involved. You'd have to make a very strong case for
hardcoding a specific implementation in the core language instead of
letting the user decide.
Doing it that way is only valid for random access iterators (or more precisely, ones which provide operator+ which usually means random access) and is vanishingly rare in my experience. I would have no problem whatsoever saying that the default definition does the obvious thing, and if you want to define it differently you just have to write it by hand.