I really do think Matthews has the ability to score 70 goals in his prime. However the reason that is so rare is not just ability but just forces outside of your control. Nevermind the shortened season, last year his first 10 games were a write off due to his surgery recovery. The season before he had a bum wrist for half of it. A few years before that he had shoulder problems, and back problems.
Chances are, he ends up hurt at some point playing the minutes he does and the abuse he takes. So I think 60 is pretty reasonable assuming mostly good health. It would take extraordinary luck to stay healthy, play 82 games, and score 70(+ in this scenario maybe) goals... and I would rather he saved that horseshoe for the playoffs