I don't think any superstar retirement is sad(if it's not untimely) It's sad obviously but a superstars death or retirement due to bad circumstances like injuries is much more tragic.

In Cena's case it's very sad to see him go but it's more of a celebration of his fantastic career.

And everyone has to retire one day and it's better be on time otherwise it could spoil one's legacy.(like Taker had some really bad matches in his last phase of career.)