The idea that a CF has to score 20+ league goals a season in order for you to win the league somehow gained traction in the Caf without any real merit to them. For every Henry or Aguero you can easily find a Drogba or Benzema.
Look at Drogba stats, for instance. (Goals/Apps, league only)
04/05: 10/26
05/06: 12/29
06/07: 20/36
07/08: 8/19
08/09: 5/24
09/10: 29/32
10/11: 11/36
11/12: 5/24
He won the league in his best scoring season (edging out Rooney, just, but we really threw that away with a 1-1 draw against relegated Blackburn, losing out by 1 point at the end), and two seasons when he scored a respective 10 and 12 league goals. And that season was really an anomaly as Chelsea as a team scored a whopping 103 goals when they average for every other season during that time would be 68. And the consensus is Drogba was a brilliant CF and one defenders hated playing against, and was the mainstay in that team even as they brought in one striker after another, with absolute big names like Shevchenko and Torres.
Another example is Zlatan, he scored 16 goals in 35 league games in his first season at Juve aged 23, 7 in 35 aged 24, won the league in both, didn’t score more than 20 league goals until age 28 playing at Inter, and didn’t score more than 30 goals a season for club until age 31 playing for PSG. He was one of the most sought after and highly regarded forward in the game throughout that period.
If Martial ends the season with 2 league goals scored, sure, it’s unequivocally bad, but if he scores another 11/12 goals in the next 22 games and we end up winning the league or mount a very strong challenge until the last couple of match day, does that mean he needs replacing and another CF would have propelled us to the title? I would say that argument is based purely on projections and not real evidence.