Turning all Ailment Effectiveness into ailment penetration is a big loss, because it breaks the interactions of ailment builds with mountains of nice existing content. The issues it creates will plague future content too.
Penetration & shred is tuned around the idea that it’s a scarce endgame multiplier. This doesn’t fly at all when your main scaling mechanic has you drowning in a few hundred % pen. Ailment builds are left out in the cold, with loads of formerly exciting content now super lackluster and also getting starved out of interesting ways to scale their damage at the high end.
Converting ailment effectiveness into +Base Damage to <Ailment> rectifies this issue while still achieving most of the stated goals of the rework, of improving clarity wrt. relationship with other damage scalers and ditching multiplicative scaling for shred and other secondary effects. The symmetry of using flat dmg to scale both hits and ailments should also make it quite a bit easier to intuitively understand and compare different damage sources.
There is one stated goal of the rework that this doesn’t address: clarifying that penetration applies to ailments. I think this is okay, since it’s not clear that the existence of ailment-specific penetration will entirely resolve this problem either. If someone starts with the assumption that general pen stat is for hits, showing them that there’s a separate “pen for ailments” is only going to reinforce that misconception if anything. At the end of the day it’s a genre where ppl will need to check their assumptions every now and then - you can make some things simpler, but beyond a certain point trying to engineer out every small ambiguity is a Sisyphean task with greater costs than benefits.