Going to the Tigers when he did was probably the reason JD is what he is today as a hitter. Working with Cabrera and Victor Martinez had to have made an impact on his approach to hitting. The deep breath before every pitch is straight from the Martinez approach at the plate.
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.