I guess we have different opinions on Mats then. The guy is mentally finished and seems to turn into a 3rd division GK in high pressure games.
Edit : One more thing I would like to add is what Khaled and DonAk posted in the Donnarumma poll in regards to how Mats salary will increase as the years go by. So even if you think Mats and Onana are somewhat equal ability wise it would still make sense to get Onana on the cheap and bin Mats who supposedly will end up earning ~15M net.
I don't disagree with your opinion on ter Stegen, but I don't think Onana is the solution either. He's a Navas type of goalkeeper. Those rarely become WC.
Sure Navas reached his peak around 30 so there's still time for Onana, but I can't see him becoming a reliable GK.