And last I checked, removing software that blocks the radio function would not add a cubic nanometer to the total volume of the chipset.

Apple has already done away with the headphone jack, and re-enabling use of the headphones would probably mean having to add a receiver to the tiny circuitry in the plug that houses the DAC. That sounds like a lot of work for a niche feature, and it's not going to help users of wireless headphones either.

...until wireless headphones started becoming common.

Then I'd expect the FM reception functionality to be built into the headphones, with the phone acting as a remote to switch stations etc.

Not that I'm for eliminating the 3.5mm jack: this is just one among other reasons I'd refuse to get an iPhone even for free.

Or using the phone's internal speakers.