You clearly haven't played enough. Sentries are extremely vulnerable to an ubered heavy, and to demomen and soldiers if you just pop around a corner and let off a shot before ducking back. Besides that they have a limited range, so you can only use them effectively in chokepoints. Scouts can't take you out "before you know you're being attacked" unless you're already low on health -- and as it stands scouts are only good for 1) rushing, and 2) attacking isolated units. It's all about the team play -- scouts can't do anything against groups, and sentries are much weaker against multiple opponents.
Everything is extremely vulnerable to an ubered heavy. Heavies are supposed to be for defense, though, so don't you think there's anything wrong with the fact that a defensive + support class can take out a sentry gun in the other team's base? The class roles don't seem to fit. I'm all for thinking outside the box, but a soldier as the primary "heavy" ranged class should be able to take out a sentry which currently isn't possible if the engineer is there (which they should be).
In terms of ducking around corners, sentries lock instantly now. So if you are going to try to take out a sentry without a medic and the engineer is next to his gun with a dispenser, you won't take it out. Engineer heals 1/2 the sentry hp per swing at a cost of like 20 metal or something. Meanwhile you are taking damage and without a medic you are going to die first. The other big change to sentries other than instant target lock is they completely block all damage to any targets behind them, so as an engineer you can now duck behind them and receive 0 damage from a rocket that hits the sentry (TF1 the rocket would damage you and push you away).
Not to mention, at certain choke points (say the ramp room) range is not an issue.
In terms of a spy sapping the sentry, again, put the sentry near a wall, throw a dispenser up, then camp it with your back to the wall. A spy can't sap it to death and you can shotgun him/repair your sentry if he tries, and with your back to a wall you won't get backstabbed.
Demomen are more of a problem at close range (long distance sticky bombs are not frequent enough to kill you or your sentry if you're repairing) but again certain places like the ramp room in 2fort are not conducive to the demo taking a sentry out from any of the possible entrances.
Yes, teamplay can counter a single sentry gun but then if you add another person on defense the teamplay is outnumbered again; my point is it shouldn't take 2 people to take out a sentry gun.
And yes if you get shot point blank by the scattergun by a scout, you will go down in 2 hits. Baseball bat is about 3-4 swings or less if they get crits, not to mention past 2 or 3 hits you can't do much against them with the rocket launcher without hurting yourself.
In general, not just with the scout, the lethality of all classes has been increased especially at close range. The scattergun is more brutal than the shotgun but they're both capable of taking out the heavier classes in a few shots, which IMO doesn't seem very balanced.