While the GPL protects the code, it does not protect the MySQL trademark. And MySQL's main value today is a positive brand recognized by many decision makers all around the world.
Forking MySQL if Oracle mismanages it is easy. Building another brand with the level of acceptability MySQL has today is a multi-year risky project.
Oracle could effectively kill the competition MySQL offers it in the conservative wealthy Enterprise market by restricting use of the MySQL brand. And this is where MySQL has been hurting Oracle most (cash cow of corporate customers that use oracle everywhere from big DBs to small because that's the only solution that got approved at all levels. That is, until the MySQL name managed to carve a place there too)