If we had to rank the top 3 United keepers over the last 20 years everyone would agree on Schmeichel and Van Der Sar 1/2, but maybe not in that order. Dave 3 for sure. Onana doesn't come anywhere near those 3, but we were told at the time we brought in Onana that we needed a "modern keeper" whose abilities on the ball would improve our buildup play, leading to better quality chances.
But that never happened. Onana's first touch on the ball is without question outstanding, his actual passing ability is no better than De Gea's passing ability -- De Gea's first touch was atrocious. And De Gea's shot stopping, even in his decline, was better than Onana's shot stopping ability.
With the end of Dave's contract coming up it made perfect sense to look around for better options, but Onana was not a better option than Dave. If we had to get rid of Dave we should have kept Henderson. And if we had to get rid of Dave and get rid of Henderson (because of the bitter interview he gave) we should have kept Kovar. True, Kovar had never made a first team appearance but he had and has all the tools we expect of a modern keeper. Would Kovar have been brilliant for us? I have no idea, but he wouldn't have cost us anything and could not be worse than Onana, whose passing is average at best, has no command of the box is his shot-stopping is not that impressive, particularly on shots that require a dive to save.
We're a badly managed club and even if we can attribute the problem children of Greenwood, Rashford and Sancho directly to them and not the club, the dysfunction of the football management of United (not the Glazers...that's a related but different problem) is best exemplified by the buys of Antony and Onana, both players who had a previous relationship with ten Hag and had one or two amazing performances for the prior clubs but have been flops at Old Trafford.