This is an excellent question. Of course, as you are aware, this quote comes in Chapter Eight, when Ponyboy is basically having to confront Johnny's inevitable death. This is indicated by the doctor's comment of "He's been asking for them. It can't hurt now." Note, too, how when they Ponyboy and Two-Bit begin talking to Johnny, Ponyboy notices that Johnny was "as pale as a pillow and he looked awful." Thus what Ponyboy says initially as a kind of automatic remark to cheer Johnny up becomes something much more significant.
The quote you have identified does two things: it firstly identifies how tightly the gang is bound together. Ponyboy is perfectly right. The gang in its present form cannot survive without Johnny, as Johnny is part of their "family" and not having him there will never be the same again. Secondly, consider how Johnny is described throughout this excellent book. Johnny's "big black eyes" are constantly referred to, and quotes establishing his sensitivity abound in the text:
Johnny was a good fighter and could play it cool, but he was sensitive and that isn't a good way to be when you're a greaser.
In a sense therefore the gang needs Johnny precisely because of his sensitivity. He is able to bring that to bear for the whole gang. Losing Johnny would mean losing that sensitivity.