Given this, i always tend to put me in Blizz shoes and ask myself "how the ability can be rewoked without breaking other mechanics?" And my idea (while being very raw) is basically a way to control BG stacks and have a meaningful way to use them without interfering with RB; i'm pretty sure we won't see RB going away at any time, so changing BG sounded the better idea.
Your iteration of my concept is surely better and i like it a lot - never pretended for my idea to be final - but the core idea (though very simple) is still there; as for SnD and Rupture, my idea was that on a sustained damage environment, you would be using the boost for them, and when burst is needed you would use the on Evis. Anyway, removing SnD from it sounds even better so you have a clear choice between 2 finishers. How does this sound?
Why i don't like BG? Because it fails to serve any actual purpose atm. Maybe it's not its fault (read: it isn't), but the best way of intervention is to change BG since RB is so deeply tied to the spec that any change is more likely to result in a broken spec than in an improvement.