But this (and similar) doesn't actually answer the question, i.e., this would really only explain why you would generally expect oceanic lithosphere to underthrust continental lithosphere, not why you would expect oceanic lithosphere to be able to subduct, i.e., sink into the mantle. Similarly, this explanation would not really help to explain how ocean-ocean convergent boundaries work. The key is that the subducting portion is denser than the underlying mantle, and so in the simplest sense, the density of the overriding plate (i.e., the non-subducting one) doesn't matter for subduction to be able to occur (though it matters in the sense of which plate is able to subduct, at least in the ocean-continent boundary example).