Bro, don't get upset over something so basic. Why would he word it any differently when Coby's the only future that got captured? Only Coby is mentioned because that's who they're there to save. That doesn't mean Coby's the only future for Marines. That's a dumb preposition to make.
Because it called English. That's why.
You are suggesting something that logically makes 0 sense.
If someone kidnaps zoro and luffy saves him. If luffy says "he the one that's going to become the world's greatest swordsman " are we supposed to conclude that all members of luffys crew are swordsman and that he only mentioned that in reference to zoro because zoro at that time was captured? No. That's dumb logc
It's why your points makes 0 sense.
If oda wanted to portray what you are trying to say he would have worded it" you captured koby. He is a member of sword that are going to be the future of the marines". However oda deliberately did not write it that way