Onana is, on the whole, nowhere near as bad as people make him out to be across a full season. His most critical weakness (bottle job in important matches) is mitigated a lot by the fact we're not in Europe this year. He's not a title winning goalkeeper, but we can spend the money much, much more effectively further up the pitch. Even if Onana had been the top performing goalkeeper in the league last year it still would have only saved us 6-8 goals. We still only scored 42 goals in 38 matches and were mid table for xG.
If we get a big offer in for him from SA, sure. Otherwise every penny we have not spent on our attack and midfield is money we're not getting proper value from. Onana can wait until next season.