Because most pitchers in both leagues rarely go past 5 innings, the NL really has no excuse now for not adopting the DH. Nobody, except his parents, wants to watch a pitcher bat.
Every professional athlete has the right to retire whenever they want. The way Luck did it was very shoddy. He will be considered by many Colts' fans as a quitter and that label will override all the good things he did as a Colt.