The UN only survives because it is toothless, and the permanent seats of the UNSC are essential to guaranteeing buy-in from members that otherwise wouldn’t allow it to exist.

It's almost as if the UN is the creation of the Great Powers, as a meeting ground where they can coordinate their actions, and is not intended to be some authority that tells the great powers what to do.

I don't know where these people learned alternate histories of the UN as being some kind of force that can keep nations in check.

It was never intended to play that role, and the UN Charter forbids it explicitly.

The real problem with the UN is that it is obsolete even for the role it was empowered to play, because with the advent of videoconferencing and abundant communication, it simply has no meaningful role to play. What it has become now is a jobs program where well connected people can obtain diplomatic posts and party in New York while not paying any parking tickets. That's literally all the UN is right now.