Why would one state leaving dissolve the union for all? When the Southern states left the union, they never even suggested that Wisconsin could not maintain whatever kind of relationship it wished with Rhode Island. That was entirely their business.
Because we fought a war to stop it from happening.
I'm talking about if we let one go unhindered, then the principle that Madison postulates that if equal states decide unilaterally to leave, then that implies the power of other states to kick out a state (the mass seceding from the lone). That would lead to tyranny when a majority of states chooses to oust a minority state (e.g., abortion states banding together to oust a pro-life state).
What I would support is a Constitutional amendment reversing the process, that is, a process for a state to appeal to Congress for permission to leave, requiring the same vote as required to join.
-PJ