Look at last year's era+. The Reds had the best in the majors last year (127) with, basically, the same pitching staff as they have now. The next closest era+ was Tampa with a full seven point drop-off. Whether Chapman is in the rotation or the bullpen, this pitching staff is a force to be reckoned with.
I'm not saying that keeping Chapman as the closer is the right choice. But from what I'm reading, many seem to think that unless Chapman was named a starter, then suddenly this staff becomes weak. That reaction just ignores the stats, and is a complete overreaction.