Onana's kicking is clearly much better than DDGs, it's his goalkeeping hasn't been good enough in his season here.
I agree that the DDG situation wasn't handled well by the club (the original contract offer should never have happened), but what you're suggesting was never an option. DDG's original offer was over £200k per week for a longer period, he refused a lower offer for a shorter period. You can prefer an imaginary scenario, but it was never an option. We either kept DDG around for at least a few years as the highest paid goalkeeper in the country, or we brought someone else in, those were the options available to us so to me it's fairly clear that the latter was the correct one. Whether the replacement was the right one is a separate debate (which isn't much of a debate at the moment, admittedly), but it was great to see us start moving on from overpaid long standing players.