If you thought we brought Lukaku for 75 to just to be on the bench, as many of us would see it as bad business. However, let's get to the facts, Lukaku is really monster striker that can draw out more defenders and dominate in ariels and could score a lot of goal coming from the header if we get wing/fullback that can cross with high accuracy rate and is very powerful but is martial kind of striker like that? Bale or Ozil on the left wing? They can't perform so well on the left wing and not good as on the right wing.
Don't get me wrong. I really do rate Luke and there's few that would be happier than me if he goes on to have a legendary impact in a red shirt. To get the best out oh him, as you rightly mentioned, you need accurate crossers of the ball and lots of creativity that he can feed on. I also observed that he is working really hard and improving his all round game, not least his defensive contribution.
My point though from a tactical stand point though is that a 4-4-2 naturally would be a more lethal attacking option given that there are 4 attacking players albeit with less cover in the middle without a second pivot, compared to a front 3. And so most times when you see a 4-3-3 type attacking line that produces goals for fun, there is a common denominator from Madrid's trio with Benzema often going wide and Ronaldo coming inside, to Barca's fluid front 3 to City's present attack and even to a lesser extent the dynamism in Liverpool's attack with Firmino flanked by pacy and technically gifted players in Salah and Mane.
I've always argued that people's criticism about his first touch is over the top but I still think he can do a lot more in areas like thriving in tight spaces then there's the issue of holding on to the ball as he tries to find teammates when a defender is tight on him with his back to the goal.
If Bale sorts his fitness issues there's no other player I'd rather have on the left or right of attack. You obviously do not remember when he tore Maicon a new one vs Inter Milan bombing up and down from the left as a left sided midfielder in Redknapp's 4-4-1-1. He was very unpredictable because he could cut inside and shoot or stay wide using his pace and deliver crosses into the box. Before he moved to RM, he made a name for himself on the left side of a 4-2-3-1, he could also play as a number 10 where he could dictate proceedings better or even on the right. His versatility and world class output are the reason why I find it hard to give up on him. He'd be the perfect wide attacker for us if he could stay fit. And knowing that Jose has a pretty good record of keeping his players fit, I feel that there might just be a fit player in Bale somewhere.
As for Martial I don't think being good aerially I'd such a big factor. More important is his ability to link up well with his wide attackers combined with trickery, pace, exposiveness, coolness in front of goal (he's got this in spades), movement (he needs to do more in this area), etc. I also understand that Luke is showing willingness to learn and improve and I thought he had a very good all round performance against Leicester and if he continues this way he may hit the heights of a dynamic forward but let's not get carried away as bigger tests than Leicester will come.
For what it's worth I do think a 3-5-2 would be the best way to maximize the abilities of both Pogba and Luke especially if the wing backs in the system are good crossers of the ball.