I am not 100% certain Luck's retirement will be permanent. He is well educated and has traveled extensively so life without football isn't that hard to imagine for a guy like Luck.
But given that some QB's are playing well into their late 30's it could be Luck wants to clear his mind and body and who knows three years down the road the itch to play might become too much to ignore.
In other words he could change his mind down the road. It would not shock me.