Try to flank it. It turns almost instantly and begins firing into your face. Try to dash past it via Genji or Tracer - Note how during the .2s travel time it locks on and fires several bullets(which all hit you - it has 100% accuracy even on fast moving targets).
I'd like to point out your turret-destroying strategies are best-case scenarios. Remember that we also have an entire enemy team to deal with on top of this. On several maps, there is nowhere for the attacking team to snipe it without being within its 40m range. Hanzo and Widowmaker need to line up a shot/charge/etc, which is plenty of time for it to lock on and attack them. The obvious solution is to either shield them with Reinhard or have another team member take the aggro. Using your best-case-scenario logic, we have to assume the enemy team will also have a Reinhardt or other players actively guarding said turrets, or otherwise denying you easy shots at it. This means you're using at least two players to deal with an automatic mechanic created by one player. If they have multiple Torbs and a half decent team, forget about it.
I'd also like to note that as someone who plays Genji religiously, I can tell you he can't simply "E" the turret. That works against a Lv.1 turret(though his basic attack is far more effective at that point, considering its slow attack rate and the likelihood of Torb being around to repair it), but the Lv.2 turret has too much health to burst down with Deflect, especially if Torb is attending to it.
"Git gud" is never effective in an argument. In most cases it's the equivalent of saying "I can't git gud without that mechanic so you should git gud instead!". By the way, his rivet gun is very effective both in close range and long range. Firing rate is 2RPS and it does 70 damage. His molten core makes his turrets even more powerful while also making him a bigger threat. He is fully capable of defending himself without his turret in it's current state.