Who needs fancy airblasts?
Two shots from a Loch N Load destroys a sentry in a less than a second if the engineer isn't repairing it at the time.
The normal grenade launcher is too slow to do it and the Direct Hit just doesn't seem to do as well.
And in close range if you are in the right spot, the Heavy can destroy a sentry very quickly. The worst thing you can do with a sentry is take it on at long range because it still does a lot of damage compared to what you do to it. (Also any smart Engineer using the short circuit can stop rogue stickybombs)
I don't find airblasting a good way to destroy a sentry. It's just that I want to destroy one sentry with it, as if it was some sort of personal achievement.
As for the Loch N load, yes, I agree, it's a great weapon when it comes to destroy sentries, but hitting anything else with it is pretty hard, so I rely on the stock one. Also, I don't think the short circuit is that good, even in the hands of a smart engineer, because he won't be able to destroy projectiles and repair his buildings if, let's say, a demoman/soldier and a heavy attack at the same time.
Of course, on both scenarios (Loch N load and slightly organized rush) it's like the engineer's team is not helping him in any way. A good team will pretty much watch all the spots the sentry can't reach*.
*This goes from server to server, not all the teams we find on pubs are organized to do such.