Tom, just to be clear, your counterpoint to the original wording is "Recovery codes _should_ be mandatory", correct? Would you consider "Recovery codes should only be mandatory if a user has only configured a single second factor" to be a reasonable alternative?
I have multiple U2F keys configured on all of my important accounts. I'm comfortable enough in my belief that I won't lose _all_ of my keys to not want recovery codes to exist so I don't need to worry about storing them. This places me in the extreme minority of users to be certain, but I still don't want my security weakened by recovery codes that I won't ever use.
I'm ambivalent about the multiple factors case (unless one of the factors is SMS). My feeling (can't back up with evidence) is that most people who do that are savvy, but remember that part of the point of recovery codes (which are in fact a second factor) is to protect you from the service provider's account recovery flow. The more routine account recovery has to be, the less secure the service is likely to be.
I have multiple U2F keys configured on all of my important accounts. I'm comfortable enough in my belief that I won't lose _all_ of my keys to not want recovery codes to exist so I don't need to worry about storing them. This places me in the extreme minority of users to be certain, but I still don't want my security weakened by recovery codes that I won't ever use.