This game isn't ALL about 1v1. But if you are against Zed, you are in a SOLO lane. Zed is also an excellent split pusher, and it's much more desirable to only send one ally to deal with him, so you don't get jumped on by the other 4 members of Zed's team.
This game is not about 1v1s, but there are certainly instances where it is only one champion versus one other.
As for changing it to a cleanse, that only makes it useful vs Zed. If I'm Fiddlesticks, and I ult into the enemy team, but now, whoops, my Zhonya's only cleanses, isn't that something? Guess I'm dead before my whole ult can even channel. I'm Lissandra and I dive into a team to ult their ADC because I don't have to ult myself, I have Zhonya's....wait no I don't, I have a crummy cleanse that USED to be Zhonya's. Oh hey I'm Lux, I have long Cool downs, and if I get jumped on before I get them back, I'm fucked. Oh hey I have this temporary stasis I can use to wait until my CDs come back up and potentially have a chance, or if nothing else waste the enemy's time, possibly buying time for my team to get here. Or not, I can just cleanse a single spell that won't help me from getting blown up.
You are proposing a change and you aren't seeing the bigger picture here. Zhonya's, yes, can screw you if you don't use it properly, but it's also an item that enables a lot of Champions to even be played in the first place. It diversifies picks, and here you are proposing that it should specialize in countering Zed only. If you aren't skilled enough to use it effectively, get in line, practice until you are, like the rest of us do.
On top of that, we already have items that cleanse. Your support should be buying Mikael's vs Zed, and that way, ANYBODY can make use of its cleanse.