You miss the point. Without Iwo, regardless of the casuelties, we had no base to launch 1000 bombers effectively.
The B-29 missions were launched from Saipan, not Iwo. Though in theory the Japanese could have based fighters there, there were other ways to limit their effectiveness (carrier fighter sweeps, fighter escorts, and naval shelling).
The night raids on the mainland, had their defensive armament stripped, and accordingly would have had smaller crews. Even if a B-29 has augered in rather than landing on Iwo Jima, the crews could have jumped, or gotten out after ditching. They would have had a pretty good chance of survival, and could have been picked up by Submarine. There were other options.
Iwo was not, in the end necessary. Noone knew that because of the fog of war. The major accomplishment of Iwo was that fantastic picture, and I submit that is a poor payback for US lives lost.