What is the rationale for limitations on pointer arithmetic or comparison?

Question

In C/C++, addition or subtraction on pointer is defined only if the resulting pointer lies within the original pointed complete object. Moreover, comparison of two pointers

Answer 1

You only prove that the restriction could be removed - but miss that it would come with a cost (in terms of memory and code) - which was contrary to the goals of C.

Specifically the difference needs to have a type, which is ptrdiff_t, and one would assume it is similar to size_t.

In a segmented memory model you (normally) indirectly have a limitation on the sizes of objects - assuming that the answers in: What's the real size of `size_t`, `uintptr_t`, `intptr_t` and `ptrdiff_t` type on 16-bit systems using segmented addressing mode? are correct.

Thus at least for differences removing that restriction would not only add extra instructions to ensure a total order - for an unimportant corner case (as in other answer), but also spend double the amount of memory for differences etc.

C was designed to be more minimalistic and not to force compiler to spend memory and code on such cases. (In those days memory limitations mattered more.)

Obviously there are also other benefits - like the possibility to detect errors when mixing pointers from different arrays. Similarly as mixing iterators for two different containers is undefined in C++ (with some minor exceptions) - and some debug-implementations detect such errors.