I don't see how it would be impossible for mankind to survive if we preserved our countries and borders. We've managed to make it this far, right?We have to learn to live without borders. If we want to survive.
I have nothing against different cultures. On the contrary, this is one of the things we need to learn - to respect the different cultures.
As far as the different languages are concerned, all people should speak the same language. (This need not necessarily be English. I am for Esperanto.)
But all languages should be preserved. This means that most people should be at least bilingual.
(Regarding the religions - all religions should be banned. Especially the monotheistic. Religions that have many gods are usually more tolerant. (But this is just my personal opinion.))
We should not respect all cultures. All cultures are not equal, some are clearly better than others. A culture in which women have little to no rights, would that culture really be equal to a culture in which men and women are equal? Honor cultures, child marriage cultures, cultures in which animals are treated badly etc. We should never tolerate inequalities or cruelty.
As for the language part, I think that is inevitable. I think it will be English, which is widely used in business and education all over the world. Esperanto is a good alternative. However, I think it is too based on Romance languages. Most constructed languages are very eurocentric.
I agree with you on religion, although banning will not work (this will only lead to religious extremism).