For some sizable amount of the user base, assuming they can even be convinced to use a password manager in the first place, not being able to also store 2FA codes in the manager will become their excuse to not use 2FA codes.
A great expanse of users (note, not normally the ones who frequent HN) see all these 2FA codes, and passwords as well, as just an irritating impediment to accomplishing whatever goal it is they wish to accomplish at the time.
