I'm not talking about a Pyro holding you in the air with multiple airblasts for prolonged periods of time. The Pyro only needs to get you into the air once. A Pyro with Degreaser and Reserve Shooter can do the following in less than two seconds:
1. Puff flames, ~10 damage
2. Airblast
3. Quick-switch to Reserve Shooter
4. Minicrit shot, ~70-100 damage
5. Second minicrit shot, ~70-100 damage
The Pyro can then quick-switch back to the Degreaser and proceed to puff-and-sting as necessary until he has time to reload his Reserve Shooter. Even if he foregoes the initial flame burst, if he can get two shots in with even decent pellet grouping, he can do ~120-180 damage easily.
Since I play Medic most of the time, I'll just describe the sort of thing that's been happening to me for the past two days. I'm healing a team mate, and a Pyro charges straight at me. If my heal target doesn't kill him first, I'm doomed. Even at full health, he can airblast me and get two quick Reserve Shooter shots and kill me by the time I've switched away from my Medigun. I don't have time to tell my target "shit no, Pyro, turn around!" The same is true of puff-and-sting, obviously, but this doesn't require the Pyro to be within melee range.