Is there any elite modern day attacker without defensive duties?
People need to go and watch Messi and Ronaldo carefully. Take your eyes off their attacking and scoring for a whole match and just focus on them defending. You'll be shocked. It's amazing the type of shift those guys put into each game. I watched Ali (not so elite admittedly) and Kane today and I was VERY impressed by their defensive play. This idea that a player can be let off defensive duties in today's 'mass-attack - mass- defence' style of football, is unrealistic. As long as an attacker is not asked to play as a center back, he had better be ready to put in defensive shifts. It's like saying the modern day full back shouldn't go forward and attack. We already see how that is panning out for our team when the full backs can't surge forward and put into defence splitting crosses.
Watch how defenders like Pique, Ramos, etc add to their teams' attacks. As it works with defenders, so does it work with modern day attackers.
As talented as he is, Martial will just have to develop his game all round and that includes his off ball runs, positioning and defending.