Sniper schoool is measured in months, so is basic training, so is advanced infantry, survival school is measured in weeks, qualifications for various weapons can be done over days.
However, training isn't "you've done it, now you're done", it's an on-going process. Experience is what makes them what they are. You're right, RPG's don't do this very well when the fluff claims you're the "best there is to offer organization X" when you start off with, at best circumstances, an 80% success rate on a shot (meaning you're around a 40-50% under "normal" environs of combat). Absolutely, that doesn't sound like a professional, it sounds like someone who understands what they're supposed to do, but hasn't done it enough.
So, in real militaries, they have operators train constantly while they aren't on deployment. Doing this doesn't just keep them sharp, it often makes them better for the next time they're deployed. There's no equation for this in an RPG, so they go with experience points instead.
Yet I actually think this gets away from the point of the discussion. The post title reads that a sniper isn't possible in DH2.0 Beta, when just as in Only War, it absolutely is. If the post was about, "RPG's aren't true to life, so let's not talk about mechanics", then i'd be more inclined to follow your argument.
Lastly, whether a sniper or a sharpshooter is useful in DH2.0 has merit is entirely up to the players and the GM of a game. If the Arbites find the enemy, the psyker uses his voodoo to scout it out and the scum draw the enemy into position, the assassin can certainly put one between the sorcerer's eyes from 600+ meters away, using the Guardsman as a spotter. If instead the GM decides to make a large combat inside a building to be the final confrontation, the sniper would be acting as a sharpshooter instead, something they'd also be good at.