The question for me has always been the necessity of taking the island. It was argued that P51s needed the airstrip to escort the B29s. Before the island was secure the decision had been made to concentrate on low level incendiary bombing, rather than so called daylight precision bombing. This made the P51s much less necessary. The second justification for the assault was the B29 bombers that subsequently used the airstrip for emergency landings. The numbers are usually given as 25,000 aircrew saved. This demands that we accept all aircraft that used the strip as lost, if Iwo was in Japanese hands, and all aircrew as KIA. A stretch. What is not a stretch is that the US faced the assault on Okinawa less three highly trained and competent Marine divisions which were rendered unusable after suffering 23.000 killed and wounded on Iwo.
I agree that the 25-30K "lives saved" statistic is overlooking other islands, naval search-and-rescue, etc. But I also think it's a stretch that three divisions were rendered "unusable" - although your point at the degradation suffered is well taken.