why the US Navy's Ohio-class nuclear-powered submarines will be decommissioned by the end of 2028 ?

All four of the U.S. Navy's Ohio class nuclear-powered guided missile submarines, or SSGNs, are set to be decommissioned by the end of 2028, according to its latest long-term shipbuilding plan. The USS Ohio, USS Michigan, USS Florida, and USS Georgia are among the most capable and in-demand submarines in U.S. service today.