Keep in mind that balloons are much cheaper than planes to build and operate. Worse yet, the populated areas on earth are only a fraction of the whole surface area. Expect balloons to make dense clusters rather than evenly distributed over the sky. Needless to say, there will be more flights to more populated areas as well. Increasing the likelihood of collision to a level that the pilots will need to constantly monitor the skies will increase fatigue and cause indirect problems. The cost of all this will be felt by the aviation industry, and will be passed on to us.