I don't think we have to worry that much about Marner. He'll definitely work on improving his strength and with time that will keep growing. Though he showed last year that he isn't a pushover. Didn't we have plenty of pre-season videos of him even out-muscling Matthews in puck battles?
I dont think anyone is worried.
He played 77 NHL games this year, the injury that kept him out of the lineup could happen to anyone, and he contributed big time.
I think everyone just knows its his move obvious area of improvement (above his shot). He can absolutely be a top 5-10 scorer in the NHL, but to do so, he will have to become more dominant with the puck, kind of like Willy is.
His vision is maybe a tier above Willy's (who is also good), and is work rate is definitely higher - if he had the strength to match, he would be an absolute beast