What if nid reaches the maximum ?
If nid reaches the maximum your site will no longer be able to create new nodes as there will be SQL errors whenever Drupal tries to insert a new nid.

Chances are though, that drupal.org will hit this limit before you do, regardless of what you're doing to your site in any useful real-world application.

If drupal.org gets anywhere close to 4 billion nodes, there definitely will be a workaround readily available for you.