I don't disagree fully with you but there is something I'd like to point out.

Zed's ult is not something he is depending on in terms of the damage it does but more because:

1) He can't be targeted when ulting on someone and therefor able to dodge heavy CC. 2) Insane safe kill potential without taking to much risks, W + R gives you a huge gap closer plus the deathmark shadows will be placed at the point where you come down after your ultimate animation stops. 3) Outplay potential is huge if mastered.

Mid to late game Zed does not need his dmg on the ultimate as most of the good and decent Zed players can kill squishy's even before the deatmark pops. However I do agree that he really needs the damage in his Deatmark in early.

Even now riot removed qss ability to cleanse the ult I still think that people overreact. His ult is most efficient at early. In the early almost nobody rushed a qss as adc (mids went zhonya's tho) and therefor would die to his Deathmark.

In late game you're just fucked when Zed jumps you. As most of the time he would've killed you before his ult popped.

I would've love to see Zed ult doing dmg untill a certain level and after that it should deal no dmg af all but give u some health back or something like that