I know you're biased, but why not?
It makes some sense when Astral Vision is up, but not all Morrigan players abuse Astral Vision (I rarely use it unless I'm in X-Factor). 4 durability means Morrigan's Soul Fist would lose to EMD L, Hadoken, etc. Even more problematic, since Soul Fist x2 is 10 durability now, but would become 8 durability with this change, Soul Fist would always lose to Dagger Toss, Sentinel Force, Aim Master, and Greyhound.
I don't think there are Morrigan problems beyond Hidden Missiles, personally. Without Hidden Missiles and Astral Vision, I would even say she's a bad character. If Astral Vision really upsets people that much, there are ways to change the Soul Fists fired by her copy that would make Astral Vision more fair without harming Morrigan when the hyper isn't up.
Really, making Soul Fist 4 durability just means I have to spend more time superjumping to throw Soul Fists, since most characters don't have diagonal fireballs. It would just make using her more annoying because I would have to run away more to find that perfect superjump diagonal to hit people at. She's already a huge time waster as it is.
I suppose my "No" is less of a "that would ruin her!" and more of a "why the fuck would you do that?"