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

The rationale is that some architectures have segmented memory, and pointers to different objects may point at different memory segments. The difference between the two pointers would then not necessarily be something meaningful.

This goes back all the way to pre-standard C. The C rationale doesn't mention this explicitly, but it hints at this being the reason, if we look where it explains the rationale why using a negative array index is undefined behavior (C99 rationale 5.10 6.5.6, emphasis mine):

In the case of p-1, on the other hand, an entire object would have to be allocated prior to the array of objects that p traverses, so decrement loops that run off the bottom of an array can fail. This restriction allows segmented architectures, for instance, to place objects at the start of a range of addressable memory.