Thank you all for your answers.
I read that most of the technical or legal or process concerns seem to apply mostly for the compiler bits such as the grammar.
My selfish motivation is the following: with the Javac backend for JDT, we may soon want to work on support for new Java version features at DOM level and beyond (eg core.manipulation) and have them in master ASAP, and if we're to contribute for example some refactorings targeting next Java version, we'd rather not have to deal with branches and feature patches when contributing to JDT.
But it seems both are not contradictory: this would still be possible to put work regarding DOM and manipulation for next Java version in master.
So basically, let's keep things as it, and when we're to contribute stuff for DOM or Manipulation for next Java, we'll do it on master anyway. This should work for everyone without really changing anything. We may revisit this discussion when we have such "DOM for next Java" contributions coming in.
Cheers,