Air carrier flight crew reported a TCAS RA with another aircraft on a parallel approach to ONT airport.

Narrative

I am filing this report due to concern for safe operations. While established on the ILS Runway 26L approach at ONT approaching the final approach fix; the aircraft was issued a descend (fly down) TCAS RA due to another aircraft that was cleared for an approach to parallel Runway 26R. That aircraft turned to final from above and behind our aircraft. The SoCal Approach controller had earlier called out the traffic to both us and to the other aircraft. Our aircraft was not instructed to take any separation maneuvers at that time having already been cleared and established on the ILS 26L approach. Shortly thereafter; the TCAS issued the RA and the pilot flying complied with the RA guidance which commanded a rate of descent of approximately 1;000 FPM. I notified SoCal of the TCAS RA immediately after we were told to switch to Tower frequency; and repeated the TCAS RA call on check in with the tower. During this time; the intruder aircraft was not in sight; presumably still descending behind and above us for the parallel approach as the fly down command continued for some time. I could not visually acquire the intruder aircraft until it overtook us on the right; and we continued to follow the RA descend command. I then visually acquired the other aircraft overtaking us on the right on the Runway 26R inbound track. At this time; I considered it unsafe to continue with the fly down RA due to the proximity to terrain approaching 1;000 ft. AGL and switched the TCAS system to TA; as the situation matched what I have personally experienced regarding parallel traffic in ZZZ per [company] policy; 'TCAS RA Advisory.' I asked the First Officer to slow to final approach airspeed to further increase separation. Once the RA was discontinued; and the other aircraft in sight and ahead of us; the Pilot Flying then leveled off momentarily to intercept the ILS glide slope. We continued the approach to an uneventful landing. I considered asking the First Officer to go-around; but it was not a viable option to level off or climb due to the descend RA with the traffic not in sight; nor a viable option to descend further below the glide slope close to the ground; so continuing on the ILS was the safest option. My ultimate concern after the fact is that the practice of clearing aircraft to fly simultaneous parallel approaches at ONT will cause additional TCAS RA descend events in the future while close to terrain.