My theory of the megalodon?

Well people say that the megalodon is gone, which is why it's called prehistoric. But has anyone notice over time, things grow or shrink and the megalodon is over millions of years old. My theory is that the megalodon does exist, just it's one of two things, the megalodon has either at the deeps of the sea and is smart enough to not be seen by the human eye. I get that from thinking about their used to be a time when sharks walked on land I don't know how but they did! So they may just be extremely intelligent for their own sake. Or the megalodon has shrunk. So it just appears to be as a great white? No one knows for sure but personally I believe it still exist. But why hasn't anyone thought of that? Or have they?