When considering the question of what would happen if we all developed to our full potential, I don't think it makes any difference whether there is an objective meaning of the concept "good".
If good has an objective meaning, we will all discover it, and thus come into harmony with each other. If good doesn't have an objective meaning, we will all learn to get along, because our immense capabilities work better when they are not in conflict with those of others. Either way, harmony is achieved. If we still cared which way we got there, that - not now - would be the time for arguing about it...
----------
If I am not willing to give concepts an objective truth then of course I can never agree with this. However we must take into doubt every single aspect of our lives in this case, for what is then objective?
If I give concepts an objective truth, then I am constantly second-guessing myself as to whether my version of the concept is any good. I am prone to being misled by someone who claims to be absolutely sure what constitutes "good" for me. Or I may be absolutely sure myself, and try to impose my version of good on others.
I avoid this by having a conception of good that is independent of the conceptions other people have. I am thus free to be free from doubt, without being prejudiced, elitist or "fundamentalist"/"extremist".