I believe I started a similar thread about six or seven years ago.
I have long felt the best bullpen pitcher needs to be in there, matched against the other team's best, when there is a threat to score critical runs. The thrill of strategy is a big part of baseball. When to play the ace in the hole or when to bring in the big gun?
Partly in jest, I blame Tony LaRussa for developing the closer role so he wouldn't have to face critical second guessing on bullpen usage. It is easier for a manager to let the closer take the blame for not doing his job than to take the blame for employing the wrong guy at the wrong time.
Btw, I believe Chapman needs to develop consistency with his fastball command more than he needs to develop a third pitch. Great fastball pitchers are not like the guys throwing 91 MPH fastballs. The overpowering guys just need command of their fastball and command of another pitch to throw off the batter's ability to time the great fastball. Randy Johnson had his slider. Koufax his curve. Soto his change.