HIV has a very fragile envelope, which is formed out of the host cell's wall (a phospholipid bilayer). Also, the spike glycoproteins that mediate entry into new cells are themselves very fragile (a certain number of these come apart from HIV virions as they circulate, even with the full protection of the stable osmotic, pH and temperature conditions found in blood and lymph).
If these things are damaged (as they are when the virus is exposed to the atmosphere, or to enzymes in saliva), they're rendered non-infectious. So many details would have to change about the cells the virus targets, and the way that it enters them that I can't see how it would be the same virus, with the same pathogenic effect if it were somehow to become airborne. There are tradeoffs in everything, and viruses face them too.
If these things are damaged (as they are when the virus is exposed to the atmosphere, or to enzymes in saliva), they're rendered non-infectious. So many details would have to change about the cells the virus targets, and the way that it enters them that I can't see how it would be the same virus, with the same pathogenic effect if it were somehow to become airborne. There are tradeoffs in everything, and viruses face them too.