Martial is on 200-250K a week at one of the biggest clubs in the world so I would say he's made a lot of his career and is unfair to compare him with Morrison and Ben Arfa. Even if Martial moves, it won't be to Newcastle. The way you are describing talent, you're forgetting Martial has much more to his game as well. Quick one two/link up is also a talent? Ability to score by chipping the keeper? Knowing when to pass the ball? The weight on the pass? Turning with the ball? Upper body strength to hold off the defender? Skills with the ball in the air? etc.
I agree with you, and I'm not forgetting those facets of his game, I've argued many times for the things you describe as to why he should not be given up on for the past five years on this forum.
However, some of what you've described, he hardly does though, he often messes the weight or timing of his passes. I can think of 1-2 goals where he's chipped the keeper, his finishes are often very similar outside of the odd wonder-goal, and part of the reason why he's not finishing his chances as much; he's predictable..
He is a great reference point in offense when he's on because he has the best hold-up ability on the team IMO, is very strong for his size, and he is clever in his link-up at times, more often than not, so no disagreement with you there but it's not often the case.
He's also the only guy, I've said it many times, that can really dribble his way out of a situation I don't think anybody else on the team can, but even last season that was hardly to be seen, and it seems to be gone from his game in the last couple of seasons.
Your only point that I find a bit weak though is about him being on 250k is in some way a reflection of his talent, and not rather an indictment on United's decision-making, acting on their perception of his talent which I think is turning out to have been a mistake, . I wasn't directly comparing his career to that of Morisson (who's still a popular subject here, that's the only reason I mentioned him), or Ben Arfa who was more than just a brilliant dribbler himself, and considered ultra-talented at a young age, but turned out to just be very limited. I'm worried Martial is headed in that direction of promise unfulfilled.
The thing about Martial is that you know he has the top level to play for United. I think it would be stupid to sell him while his stock is low and the economy in shambles for most clubs.
Rather keep him in the squad and see if he can't find back to some form again. If this continues next season, then of course we would have to let him go.
Again though, selling him is hardly an option when he's on those kinds of wages, so there really is no choice but to persist with him and hope he turns it around.