[Nanovelich]

That doesn't happen with the Zeran racial passive that reduces WIL, or Sanctuary. You 'can't' lose skills mid-battle.

But like Sanguine Crest, those effects are mid-battle and thus have no effect on skill slots. Skill slots are determined by what your SKI/WIL are out of battle. That amount doesn't change, ever, from anything that happens during battle. If you allow Essence to contribute to 'out of battle' SKI/WIL for the purpose of skill slots, then you open the door for such a thing being changed during battle.

Either way, it's a variable bonus, again.

In battle, your STR, WIL, SKI, CEL, and DEF by 1 per 20 Essence.

It's 'temporary.' Like Sanctuary. Like Superiority. Like Nerhaven/Kraken. As such, I don't see why it should have a permanent effect.