This can't be about backwards compatibility. If it was, they wouldn't have been able to add 'yield, 'await' etc.

I don't agree the keyword "generator" would have been ideal. It wouldn't work when using 'enhanced object literals' [1], where you can declare a method without even using the keyword "function". In this case, if you want the method to be a generator, you can just put an asterisk in front of its name, so it's consistent with the "function*" syntax.

[1] https://babeljs.io/docs/learn-es6/#enhanced-object-literals