I completely agree, and I have thought of this exact mechanic as a viable option for Spirit Bomb.
There are a lot of QoL changes for Vengeance that could be made, and I wish the developers were open to community feedback. Unfortunately, however, they really do not seem to be open to much more than the simplest fixes. The reason we are at where we are now is that in the Alpha, SpB replaced Soul Cleave. People (rightfully) complained that it felt bad to sit at max pain 100% of time and just wait for Fragments, so Blizzard made the easiest change possible - it no longer replaces Soul Cleave.