Mana shield was meant to be a skill that was to be used situationally, not all the time. The nerf effectively did that.
Allow me to respectfully disagree with you on that. I believe the reason why mana shield exists, is so that it can be used all the time. But only by certain people (mages).
The feeling that I'm getting is that mana shield was used by everyone, and was universally good, and that was why it was nerfed. This is reasonable, because DevCat/Nexon have been moving towards incentivizing specialization in skillsets, so that people would actually invest in skillsets more heavily (due to more stat scaling) rather than just going a hybrid build and dipping into each skillset for their best skill.
However, what I also see is people saying that mana shield just isn't good enough, and that protection and defense are the stats which are the most important. The entire point was not to nerf mana shield for mages, but nerf it for everyone else, so that mages could take advantage of their stats from specializing in magic.
The point of mana shield is to cover up for the squishiness of a mage (low str and low hp), with their strengths (mana and matk). Now instead, mana shield munches up all the mana of a mage and leaves them helpless, but STILL requires significant investment in the skillset to even get to the point where it can be arguably be considered effective.
What I think would benefit mana shield the most is for it to be completely reworked. Instead of draining mana over time, it should drain stamina to disincentivize other skillsets from using it, and to use the strengths of the mage to cover up its squishiness, bonus protection and defense should be given, which scales with matk. Just my opinion.