By the time the US was down to defending a few fortified islands in Manila Bay they had already lost the war in the Philippines. Corregidor was not going to last any great length of time. Much of its coastal defense artillery was ill-suited to defending the island against invasion. Many of its guns could not bear on the invaders for example. One of the two 12" mortar batteries was already out of commission suffering a magazine explosion destroying the battery position. The other battery was low on ammunition having supported the defense of Bataan for weeks already.
On its own, Corregidor was finished. The Japanese could have simply waited out the defenders and let them starve.