What protects Germany and Norway isn't your voting system. As Elon says it's poor. It's the other institutions and social mores you have. In countries without those safety nets, mail-in voting is truly an open invitation to by-pass the democratic process.
So lets turn the question on it's head. If you consider the integrity of the voting system to be critical to maintaining a democracy, why would you risk not having a robust voting system - even if it looks like you don't need it now?
My definition for a good voting system is it makes it impossible to buy a vote. Clearly mail in votes don't meet that. Drop boxes need some pretty good voting place security to prevent the "I give you a filled in ballot slip on your way it, you give me a blank ballot slip on the way out, and then I pay you" caper. Computerised vote casting can make that impossible.