Playing out of the back is overrated. Vastly overrated. You clearly want to see your defenders be able to handle pressure but consistently trying to break pressures with short passing out of the back invites too much danger and easy chances for the opposition, with little gain in return. Onana's footwork is superior to Dave's but we don't have the back line or the midfielders to capitalize on the perceived benefits of having a keeper who can hit short passes quickly. Many timbers have fallen to make us easy prey for clubs like Palace, but one of them was the downgrade in the keeper, the current keeper being poor at shot-stopping, which under every paradigm of goalkeeping is a crucial attribute to be successful as a keeper.
As for Dave's contract situation, you left out a key fact -- United withdrew the offer it made to Dave once he accepted it. Even if we concede Dave was awful on the ball, that's no excuse for treating him awfully, like trash.