There should be no TA/RA inhibition below 700 feet. Instead, the system should still provide avoidance guidance to at least one of the conflicting aircraft, ensuring safer operations during takeoff and landing.
It still shows you where the other aircraft are and their relative heights, so you have SA - it’s just up to you what you do with it. I would presume that this scenario has been investigated at great length while ACAS has been evolving and it is still thought of as the better solution to have low-level inhibits, as for one thing it would start conflicting with EGPWS (which has priority) and for another they would have to totally rethink the protected volumes otherwise a descending RA really could fly you into the ground to achieve separation from a higher conflicting aircraft. Having RAs possible that close to the surface may not ensure safer operations as the risks could outweigh the benefits.
Changing systems after one occurrence (MAC at DCA in this case) without taking into account the design rationale often leaves a worse situation than the original due to unintended consequences.