Much as you say that, Evening Sky is still left without any upgrades that boosts its abilities. If, as you say, one of its abilities is giving us access to magic systems...would us not upgrading the Evening Sky itself also disallow us to have better and grander options with regards to Magic Systems? On top of having better protection/utility based stats.
And, it also still doesn't detract from how Philosopher's Wreath only powers up one other system. A system that, sure, increases the usefulness of our teammates but also delays us even further from a holistic advancement in the form of Ruling Ring.