Most people will have to sync their passwords (generally strong and unique, given that it's for github) to the same device where their MFA token is stored, rendering it (almost) completely moot, but at a significantly higher risk of permanent access loss (depending on what they do with the reset codes, which, if compromised, would also make MFA moot.) (a cookie theft makes it all moot as well.)
The worse part is that people think they're more protected, when they're really not.
Bringing everyone up to the level of "strong and unique password" sounds like a huge benefit. Even if your "generally" is true, which I doubt, that leaves a lot of gaps.
Doesn't help that a lot of companies still just allow anyone with access to the phone number to gain access to the account (via customer support or automated SMS-based account recovery).
The worse part is that people think they're more protected, when they're really not.