I think there are three areas where Onana coming in for Dave will really improve us. The first obvious one is general goalkeeping ability. With the same amount and quality of shots, Onana based on recent history will save more than Dave - although I accept this will be blasphemy to some people on this thread.
The second thing is that Dave contributed to us losing control and getting countered from his kick outs and distribution. De Gea is chronically bad, possibly league worst, at this and was a turnover machine. It was very hard to maintain control (not all of our lack of control is down to him obviously but he does contribute). Onana has a much more varied range and accuracy of passing and so the keeper gaining posession won't automatically result in opposition posession.
Finally, the most interesting impact should be one which addresses your first point to a degree i.e. better passing doesn't correlate to better chance creation. One area that we have been terrible is beating the press. Among top clubs, we have an ability far below average to do so. I would say De Gea, Varane, AWB, Casemiro, Bruno, Fred are all very poor at this trait among our regular starters and so this really affects our ability to get out of first phase possession. De Gea was obviously a complete liability under pressure but now we're moving to one of the best and most composed keepers on the planet and what it will allow us to do is to use Onana like a third centre back that opposing forwards have to press freeing space for one of our attacking players in more dangerous positions which will clearly lead to more dangerous opportunities if that forward is Rashford or Hojlund who can take advantage of these situations with pace.
A superb reply, for which I am grateful.
In terms of general goalkeeping ability, De Gea is rightly subject to criticism for his distribution, but it should not go without some comment that Dave was handicapped by defenders who aren't the most adept with the ball. Martinez stands out, but of course it took a while for Martinez to adapt to the intensity of the PL. Varane is very solid with the ball but he really isn't quite the Rolls Royce of a footballer that he's made out here to be...and of course he's injury prone. And when he's out with injury we're reduced to Maguire and Lindelof. Maguire is just flat out terrible with the ball; Lindelof isn't terrible, but he's not RR either. AWB is borderline terrible with the ball, though not as bad as Maguire. Dalot had a great first half of the season but fell off a cliff in the second half. Shaw is magnificent; Malacia, however, is still learning the PL ropes. Point being, De Gea got a lot of blame for what was in fact poor ball handling by his defenders. The ball to Maguire v Sevilla was perfectly reasonable and should never have resulted in a Maguire panic. Longer balls to Antony and Sancho resulted in nothing because Antony kept insisting in shaping to his left foot; Sancho was simply abysmal. When Casemiro was out we didn't have a proper CDM to play through and in fact until we brought in Casemiro this season United didn't have a CDM to play through since Carrick retired in 2018, possibly De Gea's greatest season.
If we're going to be honest, De Gea has had horrendous back lines and midfields to work with since we last won the PL trophy, which he was here for.
As for chance creation, the PL stats teams report that Bruno led the PL in chance creation with 119 chances created. De Bruyne came in third with 98. In no way will I ever argue that Dave deserves any credit for the chances that Bruno created any more than I would argue that Bruno deserves credit for the saves that Dave made. Bruno is his own man, full stop. But we really can't argue that we suffered in chance creation this season when the undisputed PL leader in chance creation this season was Bruno Fernandes.
What about chances converted? The follow is an undisputed fact, and not a derivative statistic: United scored only 58 goals, compared to City with 94, Arsenal with 88, Newcastle with 68 and Liverpool with 75. 58 goals scored is a horrifying number. Maybe our poor goals scored record mostly comes down to Dave's underperformance as distributor of the ball? That's not an intuitive conclusion, but it may well be the case. Let's go to derivative statistics now. City led the league in xG with 2.16, Arsenal with 2.13, Brighton at 2.00, Liverpool at 1.95, and United 1.85. So far, one could say this is proof that Dave is the problem. (I wouldn't, but some might.) Where such a claim falls apart is with the xG v Actual stat, which indicts out front line before the jury -- City with +0.31, Arsenal with +0.19, Brighton at +0.11, Liverpool at +0.02, and United...this should no surprise to anyone here who watches us closely...at -0.29. One 6 clubs had a worse xG v Actual than United.
The bottom line is that our chance creation was decent enough, but without question needs to improve (and presumably Mount will help immensely, as will an improved Antony), but what really needs to improve is our finishing on goal. Onana can fizz world class balls to his fullbacks and to Casemiro but unless our striker -- which went an entire season without a striker, which is remarkable -- is a clinical finisher on goal, we'll still be mired with ineffectiveness on goal and scraping a top four spot at best.
An interesting stat I came across is that United's xG in away games was a decent 1.68 (compared to City at 2.05, which is no surprise), yet our away record was abysmal. I don't have the official away won/draw/loss record in front of me but we suffered heavy defeats away to Brentford, City, Villa, Arsenal, Newcastle, Spurs and narrow defeats away to Arsenal and Brighton. We scored 8 goals in those 8 away games.
The recurring theme for getting rid of De Gea has been that commits too many howlers and that we score too few goals because of him. I agree with the howler thesis and in fact am board with bringing in a keeper who still has 3-4 peak years ahead of him before his decline begins, but the "Dave destroys our attack" thesis is not supported by the facts. That facts are that we created our fair share of chances but that our finishing on those chance was extremely poor, among the worst in the league. No keeper, not even prime Schmeichel, could have rescued Martial, Antony and Sancho all having very disappointing -- to the point of being ludicrously poor -- seasons.
