But "be on the same mind and judgment" about what? What is scripture actually specifying here? If the answer is "everything" then that's simply not going to happen no matter whether denominations exist or not. Paul wasn't a stupid man, so I am certain he would have been aware enough of human nature to not expect the impossible, so it must be something else, and the Holy Spirit has not chosen to convict all believers into joining the exact same religious group.
And if for some reason, everyone everywhere did join the same group, it wouldn't be two minutes before they would start forming their own groups within the group based on nationality, culture, language, dogs vs. cats, whatever.