> As long as the user keeps using a relatively stable set of devices, they will 'approximately never' be exposed to MITM.
As long as the user keeps a relatively stable set of devices and knows to be suspicious if they get asked for an OTP on a device that they know has a passkey. If they don't know to be suspicious (which let's be real, most people won't), they'll happily follow the instructions and fork over the OTP to a phisher who can use it to complete the authentication somewhere on their end.
Magic links without an OTP fallback are more secure as the initial setup process because they can't be phished unless someone's actually MITM'ing their HTTPS traffic (at which point nothing can save you anyway). A phisher can get someone to send themselves a magic link, but it's much harder to get them to provide the link to them.