Except that's really not the case. He did have the one poor season (16/17) when he first moved completely out to the left, but otherwise he's generally been good and got screwed over by Mourinho's refusal to do anything but play Sanchez on the left and Lukaku in the middle.
17/18 he was easily outperforming Rashford but it took Mourinho a while before he made Martial his first choice. Once he did Martial quickly became our best and most important attacker, but despite that got dropped once we signed Sanchez. It took until Mourinho's job being on the line in 18/19 for him to get a proper chance again and he instantly became our best attacker again and pretty much (along with Pogba) kept Mourinho in a job for two months more than he should have been. Unfortunately Martial struggled with injury and illness for the second half of that season after Ole took over so never got going. The following season Ole moved Martial back up front and he was probably our best player over the course of the season.
So really, most of those "patches of bad form" were actually him not really playing at all through no real fault of his own. Pretty much every time he did get a proper run of games he's performed well. What he's going through now is really the first time since the first half of 16/17 of actual poor form, and even now he's had the lack of a preseason and then missing a bunch of games through that soft red card. As long as he gets back into form soon (and there's been some decent signs in his last two games where he's started both looking better before dropping away again) there's no real issue there.
Individual points taken, but “generally been good” is too generous a conclusion to describe Martial’s tenure at Old Trafford. And even if that’s just about the right way to describe him, "good" is jiust not good enough for a United striker:
Mourinho was a disaster all the way around and he definitely mistreated Martial, but even when Sanchez proved to be a shambles Martial couldn’t win Jose’s confidence.
Rather than go point by point here what I’ll acknowledge is that Martial is indeed a generally good player. We occasionally see astonishing feats of brilliance, such as that goal against Liverpool 5 years ago and the goal last year (against who I can’t recall), but he looks out of place as a CF and gives up too easily on half chances. He’s clearly more of an attacking threat on the left taking on defenders than he is through the middle. However, we have a Rashford problem to solve as well, as he isn’t a natural 9 either. So we jam Martial into a position where he’s not ideally suited and chances go wasted as he just isn’t a one touch finisher. He instinctively wants one or two touches to tee up the perfect shot and that’s just not a luxury forwards are typically allowed at this level.
There is a real issue here. Tony has the rest of the season to become a one touch finisher (think Ruud, Rooney and prime Cavani) or not, amd if not, then to go back to playing on the left where he can take on defenders, where he’s most lethal. I doubt you’ll find a single United supporter questioning Martial’s genius in beating defenders on the dribble, but there are significant doubts about his finishing ability in congested spaces on the first touch.