I don't get what you mean with "changing mitosis on one card is bad mitosis is an effect of summoning the same creature from your hand". Mitosis works the same as it always has, I am not sure what your problem is here.
SoPa was already nerfed from Elements, and I am not sure if there are other SoPa combos that are as OP right now. It is true that the card might be overnerfed, but I have also heard that this nerf was not enough, so i don't really know. I think more waiting/testing is required.
And also, when you disagree with a decision, it would help a lot if you gave a concrete answer to what you believe should be done instead.