Having a heck of a time posting from my phone, my apologies.
LAX and ONT are nearly aligned east-west. I lived in Westchester for nearly 10 years, 2-3 mile miles north of the north runway. It was not unusual to see landing lights from as many as 15 planes at once while looking east.
ONT is a very active passenger and freight hub. As a bi-weekly flyer back in the day, LAX landings headed towards ONT before turning onto the LAX flight path.
In 2017, the takeoff path from ONT was changed to allow steeper northward takeoffs and slower descents. Most likely to get the ONT aircraft out of the inbound LAX glidepath sooner and allow faster insertion to LAX. Back in the day we used to circle near ONT while waiting for a landing slot. See
this for a partial reference to the change.
My point is that the area under consideration contains all of the outbound ONT traffic, plus the inbound LAX traffic. Local helicopters excluded, that's a massive amount of aircraft to sort through.