You need to score more goals but for me the real question is this one:
Is the problem that the front four is not talented enough (or does not provide enough options)? Or is the real problem that United do not have tactics and players that can adequately support the front four (ie, the fullbacks and central midfielders).
Honestly, I think it is the latter. United regularly line up with two CMs that don't offer a lot in the final third, a RB that is a liability going forward, and a fantastic all-rounder LB who is decent in attack but not a truly elite threat down the left. You can have Messi, Ronaldo, Mbappe, and KDB as your front four and they will struggle to score goals against a well trained defense with 10 men behind the ball unless they are supported by other dangerous players - fullbacks who can put in a cross and combine well to overload the flanks, midfielders who can make a killer pass to break the lines or late runs into the box, etc.
I think if you bought a new CB and CDM that were elite athletes and defenders in space - who could give Ole the confidence to play higher risk football, to send more players forward and/or to play with Pogba in CM more often - you would do more for the attack than by buying a new RW.
Like I said before in this thread, we haven't had a RW since Valencia was here. Mata strays central and never had the pace or agility to play there. Rashford can play there but is undoubtedly our best option on the left, so ideally he shouldn't. Greenwood is a finisher moreso than a provider.
You might argue that it really shouldn't matter and that the tactics should be able to resolve this but just in terms of the raw numbers everything is skewed towards the left and central areas, making the task of organising a defence against us much, much easier. We don't stretch teams because the personnel we have almost all to a man thrive in similar areas. The few times we do have someone on the right, we look much better. i.e. Rashford in the CL group stages against Istanbul and PSG at home. However, that comes at the cost of a performance drop off in the left channel as no one else in the team has as strong an on-field relationship with Shaw than Rashford, and Rashford himself isn't really much more than a presence on the right, as opposed to the devilish threat he provides when he runs in off that channel off the ball, or cuts in on the ball.
Moreover, the profile of our attackers are all predominantly focused on goal scoring rather than the blend of scoring and creation that it should be. Martial and Rashford can do both, but are much more focused on scoring. Mata is a creator but from central areas. Bruno is a dual threat with both. Greenwood is a pure finisher through and through. Likewise Cavani.
The only create-first player we have within that forward line is arguably Amad and he's still at least a season or so away from being able to contribute seriously to the team. Sancho is of a similar ilk to Amad and if we get him, we would be able to deploy a balanced team of both creators and scorers. e.g.:
Scorer (Rashford) - Creator (Bruno) - Creator (Sancho)
------------------------------- Scorer (Cavani) ----------------------------
It'll bring out the best in the rest of the attack because the opposition now wouldn't need to be solely focused on the Bruno-Rashford-Shaw dynamic, but would have to give equally serious thought to the right flank, which naturally stretches the field from the opposition and will open up more spaces for Rashford, Bruno and Cavani to do what they do best. AWB will also continue his improvement as he will finally have a consistent presence ahead of him, giving him the chance to interplay with the attackers, rather than being the spare man which he often was. He's the sort of RB who prefers having two or three passing options ahead of him so that he can utilise his clever darting runs off the ball, and even in difficult circumstances his game in an attacking sense really started to come through towards the end of the season. I think he's going to surprise a fair few next season if Sancho comes.