Note: Amazon employee, but opinions are mine.

Amazon did not replace Oracle with MySQL. You seems to talk a lot given your knowledge of how Amazon operates internally and how licensing works is... blatantly false.

All of those DB's that could "pursue" a lawsuit are built on open source licenses which Amazon is not violating.

But the most important point here: The idea that you have to write a database to be happy to be off Oracle is insane. Amazon moved to an alternative that doesn't have a deep license designed to financially screw their clients as much as possible. That's a win.