I think his (perfectly reasonable) point is that, in the long run, a natural border might make it easier to establish and maintain a legal border, and will likely favour internal cohesion by forcing connections to go certain ways.

Obviously it is not a prerequisite for statehood, which is a complex construct that goes beyond territorial claims (since Woodrow Wilson at the very least, in legal terms).

Where did I say anything about any state entity's "right to exist?"