Fair points.
I think he's a good finisher, but I would disagree that he's excellent. However, our definitions may differ. For me, excellence in front of goal means a substantial amount of goals, or a really good ratio of shots to goals. I actually don't know Martial's goal scoring ratio, though.
Is his crossing underrated? You probably see more football-related posts than I do but it's been to my understanding that his crossing is okay. Not bad. Not great. Though to be fair, I never have any confidence from any of our team's crossing.
As far as his decision making goes, I'd still say it's not great even if he has improved it. I still find myself and others being frustrated by him this season. And comparing to Rashford, who is also not known for good decision making, doesn't really help his case for being excellent in that department.
So of course, at 21, those are aspects that can be improved on. But that requires having that winner's mentality and putting in the grunt work to accomplish that. Many here see the potential in Martial that he could become one of the next greats in a similar manner to how Ronaldo when he was here, but can we honestly see Martial using the chip on his shoulder to put in the work? Difference was that Ronaldo worked nonstop to improve his game and has done so every year since he was a teenager. Even when he has achieved greatness with us, he still put in the work to reach further, which is why we see him so successful at Madrid even at his age now. But for every Ronaldo, we have many cases of Januzajs, Zahas, Wilsons, Depays, Machedas, and so on. The way posters go on about Martial's ability makes it seem that he is poised to be the former and not the latter. I agree that the potential is there, but I don't agree that he's got the willingness and the "spark" to take it to the next level. There are many other exciting players out there who are either established or have much potential who can contribute just as much or more than what Martial has done. If Martial's potential can fetch us a great fee, then I say let's go for it, cut our losses, and continue on. Based on recent history, we've not missed many young players that have left us, except maybe for Pogba.
edit: Just want to clarify that I'd be happy if we ended up keeping him. Just that I don't see it being so bad if he goes either.