It’s just another ramping mechanic which means you need to nerf base damage proportionally to balance peak DPS on a raid boss.
So lets say in the first tier we’re averaging 5 casts per voidform, and the average buff at any given time on a Patchwerk is working out to 2.5 (this is a very napkin math example).
Then what you are saying is, that we need to nerf Shadow’s base damage by an additional 12.5%, to balance out the additional 12.5% peak this causes in a prolonged boss fight.
This runs into the same problem that existing ramps cause. Let’s say it takes you 10 seconds to get into Voidform, and ~15 seconds (average) to reach the breakeven (2.5 stacks) for the additional ramp that this spell would create.
That means, that for all encounters less than 25 seconds in duration, you are doing even worse damage than Shadow already does on Shadowlands Alpha. If the encounter continues up to the 5 cast maximum you might get in BIS raid gear for the first tier, then this ramp doesn’t reach its peak until 40 seconds into a fight.
For all encounter durations less than this duration, this ramp makes Shadow worse. Now, next time you level as Shadow, do daily quests, world content, legacy content, or Mythic dungeons, check your damage meter for how many encounters end in less time than that breakeven duration.
This spell, like all such ramps in the Voidform model - is a straight nerf for all that content. It makes us worse 95% of the time to make us equal on the 5% of encounters where boss mobs don’t fly away, or phase transition, or CC you, or give you something else you need to do than stack your spell on cooldown.
All ramps are bad. All ramps must go. A spell that is castable on the move, or an instant cast spell in our rotation, or a mobility spell, are necessary to make Shadowlands Shadow functional - but another ramp is not helpful.
Edit: It’s worse than I initially thought.
The nerf to base damage would need to be even more extreme than this. Because we would be synchronizing all other ramping mechanics, and any cooldowns we have such as Shadowfiend and Power Infusion, and burst phases of each encounter - to overlap during a long Voidform. This means the multiplicative effect of this ~25% damage buff wouldn’t be 25%, but say, 50% or more potentially in ideal cases.
That is what Shadow will get balanced around. We will stack this up as much as possible, slap Bloodlust + Power Infusion + potions + borrowed power systems + Shadowfiend + new ramping mechanics they didn’t listen to me about + Lose Yourself together, and the effective bonus from this will feel like +70%.
So the devs will respond as they consistently do to ramping mechanics - lower the starting point, since the end point for the ramp is fixed at ~average DPS for all specs. They will nerf Shadows base damage by 35% potentially.
Oh also, all ramps must be nerfed each tier - as longer voidforms will make this ramp even higher - will mean we need to get nerfed by say, at least 10% per tier again, because if we’re getting 1-2 more stacks from longer voidforms each tier.