Supernova had too strong of a hold on the upped metagame. As such, the viability of other decks was too low. Supernova bows are too fast and flexible.
Agreed. However, I believe other nerfs are more intresting and viable: this one means you aren't able to build Supernova bows AT ALL. You are way better off with other cards and QTs as main producers, effectively killing the upped version of a card. This is bad.
Some intresting ways of nerfing Supernova I have seen are:
1) Cost of 3
- hinders but doesn't kill the ability of chaining Supernovas.
2) Same as Nova with some (4? 6?) extra random quanta gained. Effectively the same as Nova (which is definitely more balanced) but with a strictly better effect to justify the upgrade. You might even want to nerf Nova as you did (cost of 1
) alongside this if you believe it would still be unbalanced.
Both these nerfs are less drastic and while they do kill Supernova Rainbows they mantain the possibility of using them, and the possibility of using Supernova decks in general, viable in the metagame.