You called out specific scenarios/individual at bats. There are 9 guys in a lineup, you can't pick who is going to come up in a key situation. One guy doesn't solve a (hypothetical) overall problem. Corbin Carroll in the three hole doesn't solve Dansby striking out with the 5 and 6 hole hitters on base.
Carroll would replace Happ(?), per FG would give you an extra 1.5 wins the rest of the year. A theoretically healthy Tarik Skubal (using Christopher Sanchez's ROS projections) gets you an extra 3 wins over Taillon, who has been simultaneously healthy and terrible this year.
Since the beginning of 2024, the Cubs are 4th in offensive fWAR, 6th in wRC, 2nd in defense, 10th in slug. Over that stretch, we're 21st in pitching, 11th in ERA, which follows that the ERA is propped up by the defense. Obviously in a fictional Corbin Carroll world the defense is fine, but it seems significantly easier and more productive to fix the pitching.