"Best Pitcher of All-Time" does not correspond to relievers. Rivera can easily make a bid for "Best Reliever of All-Time" but to compare against starters is not serious conversation.

If anyone needs clarification, they may conduct a salary comparison between relievers and starters.

In 2019 relief pitching is still a "fall back" for pitchers who cannot sustain the rigors of being a starter (including Rivera). This does not mean that they do not undergo quicker specialization but they would all be starters if they could.