When Usopp joins the crew he says he sant to be captain and Luffy of course declines. Usopp says fine I'll let you be captain but of you ever fail to do your duties I'll step in and take over. In Enis Lobby for the first time Luffy lays defeated in front of an enemy and could not move. Usopp stepped in and challenged Lucchi saying he would destroy him. Gave Luffy an epic speech and secured the win.
Verbal agreement: check
Both ends held up: check
Usopp original design as first mate:check
Deleted scene of garp telling Usopp to take care of his son: check
Zoro useless: check
Verbal agreement: check
Both ends held up: check
Usopp original design as first mate:check
Deleted scene of garp telling Usopp to take care of his son: check
Zoro useless: check
the truth is usopp/ blackbeard was suppouse to be firstmate make the same promise. Get in a fight on water 7 and go from duterprotaganist to antagonist. Luffy's final fight was still going to be blackbeard but it was going to be his former first mate. And instead of killing thacth he probably would have killed one of Luffy's crewmates.