In 1990, we had the largest fleet in the world.With 15 carrier battle groups, four battleship surface action groups, and over 100 attack submarines.
The nuke powered subs had a service life of about 20-25 years and could not be extended. When their time was up many were decommissioned with no replacement.
Congress, POTUS, Sec of Defense, and Service Secretaries of the post-Reagan years basically let things start declining and none of them had the ambition nor will to stop the gutting of our military in either party.