The odds of any particular mutation are small. The odds of any mutation are high. The odds of a meaningful mutation, somewhere between the two. The fact that we're seeing more spread than usual is already some evidence that this strain has some relevant mutation - though I don't know enough to quantify how much evidence.
No I don’t worry about AIDS becoming airbone, but when you look at the historical record of pandemics they are much more a problem than would appear of first glance. Pandemics that wipe out 1/3 to 1/2 the population and that appear out of nowhere are far too common over the last 2500 years for us to be complacent.