As I understand it, the UN's charter doesn't have a mechanism to strip a country of its veto power, and any amendment to the charter can be vetoed. There's no incentive to relinquish veto power, thus it's a ratchet -- it will only ever move towards "more countries can veto."

I don't think there's going to be a push to grant any more countries veto power, since vetos greatly hinder the usefulness of the security council. Certainly not a "full" veto, equal in breadth and power to the P5.

I think that if there are significant changes by 2050, it will be through the formation of a new intergovernmental body that usurps the UN in relevance. The UN needn't ever go away, but there's no reason it will always be the premier organization.

That said, it's ultimately just a political tool. If one of the P5 were to become a pariah without allies in the council, then I'm sure they could be stripped of their power or kicked out. It may not be "legal" according to the charter, but a charter is just an agreement. What matters is enforcement, and if you're being kicked out, what are you going to do about it, go to war?

@DanHomerick Yes, I think its very unlikely that members lose their veto power, there may be some legal loopholes like an Emergency special session of the United Nations General Assembly. But I think they would only kick a member from the concil after another world war.