When the oceanic portions of lithospheric plates subduct, they are believed to be destroyed in the asthenospheric mantle. They don't survive the process. The oldest oceanic crust is believe to only date from around the Jurassic period -- about 200 Ma. New oceanic crust is generated at divergent plate boundaries such as mid-oceanic ridges. There are believed to have been a number of supercontinent cycles in Earth's history since the time that plate tectonics "switched on" about 3.1 Ga.