have to leave the country to renew the visa etc etc
But you only have to renew the visa if you leave the country.
Confused? The visa in the passport is not directly connected to being lawfully in the U.S. If your petition expires, the employer can file for an extension. If the petition is extended, the alien is legally present, despite having an expired visa. The visa is only relevant when applying for admission in a port of entry (read: when coming back to the U.S. from a foreign trip).
In short, if the alien is not planning any trips abroad, the visa expire without any problem.