<<Quite easily. Two run game, "not so successful"= at least one run,>>
Not necessarilly. At that point in the game, he should have guys at the ready to come in, and watching his guy on the mound very closely. Two base-runners with less than two outs. No runs in. Pull him. Bring in his next pitcher depending on the situation, ie leftie vs. leftie, righty vs. righty, and/or play the percentages of how successful his guys are vs. the guy coming to the plate.
The plan is: Look at who is coming to the plate the next inning, and determine who is likely going to have the most success against the lineup coming up up to five hitters deep in that inning.
Obviously, that's just one scenario. If he's got a man on 2nd with one out, setup the double-play, and bring in his best chance to get it. And so on.
You've created a single scenario to make your point. That's fine - but there are many things that can happen in an inning whereas a pitcher gives up no runs, but it's time to pull him.
That's leaving a guy in who was successful until he's not so successful.