Do you people seriously have that much trouble facing engineers and their sentries? The demo man class dominates turrets with their sticky bombs and because a sentry is stationary the soldier can often fire several rockets into it at range before the turret can detect you (The splash damage from the rockets will cause a good deal of harm to an engineer if he happens to be obsessively beating the sentry with a wrench as well.) Additionally, a turret's accuracy is actually pretty bad at long distance and the rockets are ineffective. In tight and cramped areas a spy could also run into a sentry farm and use the sapper on a couple turrets then maybe even backstab an engineer before being detected if a diversion is provided to draw away any protective units like pyros. The knockback is pretty dumb though, I can understand if people want that toned down.
So what you're saying is you want the turret to be named your nemesis just to make you feel better about being killed by a guy who was standing still, whacking a sentry with his wrench. Get over it.