Would Corrosion overshadow other forms of PC / earthquake?
Possibly, though it does cost more than earthquake and the stack will still generate quanta while corroding, while earthquake kills three outright. The main point of earthquake is to get in early to harm the opponent's chances, and having to wait for that could damage your chances. Because of this, I don't think it would really overshadow it; it's just a different card with different uses. Out of the two, earthquake would probably still be more popular in rainbows, because there are more good death cards for rainbows.
Also, as Higs said, and I mentioned in my review, Death is one of the last Elements you'd think of to give such a powerful mechanic. And again, zanz doesn't refuse cards. He just doesn't add them regularly. He has his reasons for the cards he adds, but he doesn't explicitly state them.
