Why did the US Navy retain the Iowa class so long?

Was there a lesson behind keeping the Iowa-class battleships on the register until 2006, or was it only about money?

Their heavy guns still offered useful and economical amphibious fire support, a capability the Zumwalt programme was meant to replace in modern form.

Battleship guns retained a useful operational niche even after the ships became expensive to keep. Amphibious forces facing a peer opponent need sustained firepower close to the landing area. Carrier aviation can provide it, but repeated air support is costlier than artillery while targets remain within gun range. The Zumwalt programme's land-attack ambition is interpreted as an attempt to replace that capability with a smaller modern ship, an effort undermined by immature railgun plans and the Advanced Gun System's outcome.