Submarines otherwise sent to Western Australia could operate from Singapore against shipping in the South China Sea and Dutch East Indies. A force of fast capital ships, a carrier, cruisers, destroyers, and submarines makes the historical cruiser-led invasion groups inadequate. Japan might still seize Borneo and Celebes, but submarine pressure would make holding and exploiting them much harder. Operations against Sumatra and Java would likely wait until the main Japanese carrier force could support the advance.
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Singapore becomes an Allied submarine springboard
Keeping Singapore open places Allied submarines and a reinforced surface fleet across Japan's East Indies invasion routes.