Here's the problem. Zed has about the most counterplay to his kit that this game has seen. Even with his current kit with the starting shadow, he already falls off late game. Yes, he has 25% additonal AD from his shadow, no this does not mean he's meant for the late game. Late game, there's almost never a -good time- to ult. Everyone is grouped, everyone is waiting and ready for engages, this is why you don't see Kha'zix in the meta anymore. Too much grouping. So, remove Zed's beginning shadow and you have the conundrum of there never being an ideal time to ult, unless someone severely misplaces himself. Most of the time Zed is forced to use his W to be able to even get in range to ult. Then, the champion might have a zhonyas or a QSS, so already that's a lost assassination. There are so many factors that contribute as to why Zed needs an option to get out.
Yes, you can look at my match history and guess I main Zed. Yes, you can call me biased and say that discredits my word which is fine, it doesn't. Zed is squishy, you CC him during his ult and obliterate him in one big collapse. Happens all the time. If his assassination fails and he gets away, big deal. You shouldn't feel entitled to 300g because he failed to kill you. It's now effectively a 4v5 until his ult comes back because Zed without an ult is just a Q and an E every 5 or so seconds.