Modern football requires a goalkeeper who can build attacks from the ground with his accurate passing. But what happened to Machester United is that they neglected the things that a goalkeeper should have, namely good reflexes and also a good reading of the direction of the ball, and that is now not shown by Onana.
I would largely agree and say the same thing about Manchester United, which is that they have made a mistake in discarding David de Gea, although his weakness is not having good passing, in my opinion it is a weakness that can be corrected.
Build up isn't a must for goalkeeper, it's a must for defensive midfielder. Manchester United don't have enough good defenders to use that strategy, even Bayern Munich that having strong defenders yet still struggle when they have a bad goalkeeper.
Ten Hag don't have a vision, he don't know which player is good and fit with Manchester United's players.
Yes, it is not a must for goalkeepers, but goalkeepers who have the ability to do so have a plus point in this era of modern football. But again I say that it should not neglect the abilities of a goalkeeper that should be, as in the points I made earlier.
Now what happens to David de Gea is, when he is considered not to have good passing accuracy coupled with their defense that is not better organized. No matter how good a player is, he will not have a good pass when his teammates cannot open up space and can move without the ball.
This is a difficult discussion, but I think that any modern goalkeeper should be able to participate in building up the game. On the highest level I think most of the teams have a goalkeeper who is able to support the team with the ability to initiate high paced attacks. If one team has that and the other teams doesn't, one attack can make the difference between winning and losing. That is why these versatile goalkeepers are among the most expensive players in the market. The value has increased by a lot in the last few years.