Its morally bad to destroy planets with innocent lives on them, especially when its donw not out of desperation but convenience, it will paint a target on humanity's back for destroying planets , when its not needed, other alien species will be triggered by how violent humanity is destroying planets just to gain a war advantage.
Avernus is a symbol of the new imperium, the imperial trust symbolizes something better a return to how humanity should have been.
And now you want to feed cultists for a powerup, It is not worth it,morally and for spiritual reasons, it is inhumane to treat others as tools to get a advantage especially when its a fate worse than death, especially when cultists used to be normal people with lives, Its like taking several hundred steps back for the imperial trust as a whole. The knowledge gain is not enough to compromise the trust's humanity and morals.