Well, ignoring the whole meta aspect of the argument that DC would happily lose Diana over Clark
Yes, let's. -_-
Bruce represents fear, Diana love and Clark hope. Of those three things, hope is by far the hardest to live without.
Eh, that's still pretty meta. In-universe that doesn't really hold. Much of Gotham looks at Bruce as a symbol of hope. Diana represents hope to half a galaxy. J'onn actually has better global recognition than Clark, who's mostly a North American homebody, and if you're in central Africa somewhere hoping for somebody to swoop out of the sky and save you, you're probably looking for J'onn, while Superman represents those pie-in-the-sky American types who spend their lives doing stuff that will never involve you. Clark as the ultimate symbol of hope in the DCU doesn't work with canon. And you can't compare their accomplishments and say one of them has done more or less or will do more or less good for the world, any more than you can compare two glasses of water and decide which is more wet. Purely objectively, neither is worth more, in-universe, than the other, symbolically or otherwise.