Regarding the gravity bows -- they would already have perfect access to the Black Hole with very little cost (3 quanta). This is not a very large cost considering your mark. This does admittedly strengthen a gravity bow (marginally), but it nerfs all the sources of abuse (how popular are gravity bows, be serious). Considering the theme of shards staying true to an element, this is our only option. Let's be real: a gravity nova bow is so very very rare, and highly nonsynergetic with a bow-set-up.
It's like saying that Shard of Sacrifice was buffed toward death-nova-bows (scorpions anyone?). This is simply not the case, and is focusing on a very small sub-section of possible uses. The problem will not arise from Shard of Focus with this nerf -- the problem will be a very common problem: nova. Considering this nerf, focus on the card, not nova. Nova has a very long history of issues, and I don't think it's a coincidence it's the gravity-nova-bow that comes up in your challenge.