While nobody is saying we don't need to sign a striker, it is a lot easier to score goals if you can play football in the opposing half of the pitch, which is something which we will never be able to do consistently with De Gea in goal because of his aversion to leaving his line.
You mention City, Arsenal, Newcastle and Liverpool - and highlight Arsenal and Liverpool - as teams that concede similar amounts to us but score more goals. I think it is reasonable to add Brighton into that list too, given the high quality of their game and them scoring 66 & conceding 45 thus far.
Now which teams do you think play the highest up the pitch? In order, it from highest line down is City, Arsenal, Brighton, Liverpool, Newcastle, Chelsea.
Where are United on that list? 16th (which in fairness is an improvement from a few weeks ago when we were 18th). Only Wolves, Bournemouth, Everton and Forest sit deeper than us on average.
Now which teams average the highest amount of possession? In order, from highest down is City, Liverpool, Brighton, Arsenal, Chelsea, United, Newcastle.
I wonder which of the 10 teams mentioned above goalkeepers average defensive actions are highest? Alisson, Pope, Kepa, Ramsdale, Pickford, Ederson, Henderson, Steele, De Gea, Sa, Sanchez, Neto, Navas. Start to paint a picture of why Brighton dropped Sanchez and why so many Forest fans prefer Henderson to Navas. If we look at that same sample of goalkeepers in terms of defensive actions outside the penalty area, Neto jumps ahead of De Gea; in fact there are only 4 goalkeepers in the entire league that do less work outside their area than De Gea - and one of them has been dropped.
I don't know how much clearer it can be that De Gea's inability to deal with balls over the top of the defence is a huge factor in why our defence sits so deep. Because we sit deep we have to work so much harder to retain possession because our play is stretched, whereas to move forward in Ten Hag's previous styles of play you want to retain possession in a relatively compact space through triangles to invite the press which in turn creates space for the wide attacking players. Which teams tend to score the most goals? The ones that have the most of the ball.
That, coupled with his inability to command his area, possession under pressure, or even seemingly keep shots out at an average rate, means it is imperative we bring in a new first choice goalkeeper if we want to see any improvements - regardless of whether we bring in a striker of not (which we obviously need to).
Just to touch further on the inability to keep possession under pressure; that in itself isn't even the worst thing. It's the kicks out when he is pressed that are horrendous. Keeping possession from long clearances isn't necessarily expected - some goalkeepers like Ramsdale and Raya don't even try to keep possession a lot of the time with their long balls forward - the issue with De Gea is he just kicks a floaty ball straight up in the air which is nearly always won by a centre back and the opposition are back on the attack. Even if we sign a massive 7ft tall striker, those types of clearances are always going to favour the centre back running onto the ball rather than a striker facing his own goal.