The thing is that we don't have a striker who will give more than 'a bit here and a bit there'. We just need to improve the situation next summer, or January if possible. Ronaldo, Martial, and Rashford all have issues, and people will get frustrated with all of them this season.
I'm not sure Martial will pin down the role. I do think people are forgetting that he has had an issue with work ethic over his time at Manutd. He was booed by Sevilla fans whilst he was on loan, due to his lack of effort. He is better at the hold-up role, but the lack of runs that he makes will be an issue. For me, Martial doesn't make the run that opens up the space for Fernandes - he has shown his reluctance to do this throughout his career. He will not press anymore than Rashford, either.
Rashford has flaws in the role, and it is clear he has never been a striker. But, despite playing out of position, he has still played a major role in our three wins. We cannot guarantee the same would have happened with Ronaldo or Martial.