I wonder if we are going to see airborne HIV eventually. Because, why not? It would be very reproductively succesful.
Eh, HIV has a very specific affinity to CD4+ T cells, which explain the symptoms associated with AIDS.
For viruses that spread either by droplet transmission or true airborne transmission, they must have sufficient affinity for upper or lower respiratory tissues. Usually, this involves an optimization problem where they develop a particular affinity for those tissues and are less effective at other tissues. They then tend to present with... respiratory symptoms.
All viruses are different, but on it's face, it doesn't make too much sense.
There are some respiratory viruses with interesting mechanisms for causing immune suppression to evade the host immune system for longer, but none that I know of work the same way as AIDs.
I suppose in a future world, you possibly could create a respiratory virus that causes a cell to create two viruses, one copy of the original, and one copy of standard HIV.
There's a lot of issues though: For one the viral envelope would have to actually fit all that genetic material inside it. For two, it's not like copying a file... The viral genome has instructions for making more of itself, and that includes a hideous protein folding and packaging problem that is really non-trivial. I don't think you could take two off the shelf viruses and just splice them together and stuff them in a virus, it would just at best create a virus that after the first generation, just created two viruses. Finally, even if you did do this, the respiratory part of the virus derives zero benefit from the HIV component, so it will, after a few generations probably just ditch that part and evolve back into a cold virus.
Everything is successful if you assume it Magically Just Works.
Viruses, like any creature, have physical limits, practical tradeoffs, and even game-theory/ecological considerations as they compete against other.