Well, solving/mitigating the ABA ambiguity can debug use-after-free errors in single-threaded programs also. Because when a pointer A is freed to B, and then recycled again for a new object, we can make it into a different pointer A' (e.g. with a tagging scheme). So then when the old A pointer copies are lingering around, we can tell they are invalid due to having the wrong tag.

Solving ABA is probably a point in favor of indices (if we are working in a higher level language) because their type supports the bit operations for tagging. However, some hardware has support for hardware tagging for pointers. E.g. ARM; Android uses it.

With indices what you say can be implemented trivially, much simpler than with pointers, by always incrementing a reallocated index (i.e. an index extracted from the free list) with the array size and always addressing the array with the indices modulo the array size.

With the array size chosen to be a power of two, this adds negligible overhead in time and no overhead in space.