Some people are willing to serve, but not in a face to face combat role, nor do they want to spend their time stationed on a big boat.
The Air Force does have troops in physically demanding, battlefield combat roles, and not everyone enters the Air Force just because it is the "easiest" of the branches of service.
The Air Force has a lot of more technical positions available, as well as a lot of good educational opportunities available.
If you get rid of the Air Force, many people who would enter the Air Force would simply not join the Army. When you join the military you don't get to pick your assignment. You can request what you want to do, but there are no guarantees.
One size does not fit all, and if you want to maintain a volunteer military, you need to give people options they are willing to accept.
That's a good point. I know a guy who's been in the Air Force Reserves for around 20 years. He may not be pilot material, but someone has to put the missiles together and load them onto the flight. Different jobs for different people. Everyone plays an important role.