'Given the size and scope of WhatsApp's user base, we feel that their choice to display a non-blocking notification is appropriate. It provides transparent and cryptographically guaranteed confidence in the privacy of a user's communication, along with a simple user experience. The choice to make these notifications "blocking" would in some ways make things worse. That would leak information to the server about who has enabled safety number change notifications and who hasn't, effectively telling the server who it could MITM transparently and who it couldn't; something that WhatsApp considered very carefully.'
Why not have every client show up as having safety number change notifications on and just choose whether to display them client side depending on user settings? i.e. if you have them off, no message will display and the message will automatically be resent using the new key?
You quoted the answer to that already. If these change notices are "blocking", then the sending device won't re-send the message until the user has verified it. If the user hasn't enabled the notification, then the sending device will re-send immediately. This makes it trivial for the server to figure out who's actually enabled the notifications and who hasn't, which means the server can be confident about when it's actually safe to MITM.
Why not have every client show up as having safety number change notifications on and just choose whether to display them client side depending on user settings? i.e. if you have them off, no message will display and the message will automatically be resent using the new key?