Ubuntu: "We have no plans to fork GNOME" (derStandard.at)

There is no bright line divide. If they want to give up on first try and move on, that is certainly their choice but if you wanted to make a feature a part of a larger project, it usually requires more persistence than that. If you can live with that divergence, more power to you. It has been my experience that downstreams sooner or latter realize that it is too much of a burden even if it does buy you some uniqueness in the shorter run.