Why would the rogue attack a swarm of 50 guys. That's 50 perception checks, statistically some will see them and get the reaction shot. Why In your hypothetical is the player brain dead? Why are there fifty guards in one location? Why wouldn't the rogue be taking advantage of the fact that every guard is now on the same location and not defending something important?

And this still doesn't get to the heart of the issue. You've done it to the players, so now they will constantly be saying "I'm ready for any and all attack, I ready my bow every six seconds to fire!" "Oh we can't do that? You did it when the rogue was ambushing those guards"

It's stupid.

This entire thing is stupid. Just let the rogue do what the rules say they can do, and stop trying to twist them to stop something that you can easily solve with tighter patrols, lighting torches on every corner, or just taking cover. Have the hunted guards act like same human beings.