Ok, I'll try to point out why your argumentation is wrong in many ways.
First off, no one talked about getting a complete Zhonya's first item, howeever, it isn't bad as you sound it is. Because, to beguin with Zed's R in laning phase has the same cooldown as the Zhonya's, I don't even want to try to understand why did you say it has 3x higher the cooldown of Zed's R, if that only happens much later into the game and when Zed has 40% CDR, but anyways, moving on to the point - Most players take inspiration as secondary run, and go for
and rush
and from that they have two options: If somehow they are losing against Zed in that situation and used their
they rush
or they can rush
and complete Zhonya's after, and if they aren't struggling, they can simply ignore Zed and proceed to rush their first item as they wish. Considering that the player we're talking about is decent, he is wasting 1100G to make Zed completly useless and being a total "cockblock" to him, and since Zed can't kill you and probably can't waveclear faster than you to roam, you can sit in the lane shoving the wave into his turrent and prevent him for moving from the mid lane.
Now moving to the next thing you said, it's actually very funny:
"Imagine playing as Zed and your first item not having AD, but magic resist. Ok I know Zhyona adds AP, but it's negligible tbh. Just imagine being behind simply because you have to build against enemy. It. fucking. sucks. as. fk.
Zed could buy hexdrinker, which is a defensive item, but first it's SO MUCH CHEAPER, second It already has strong defensive passive in it, third it builds into very strong all-round item."
If people here think that mages lack of build diversity - I introduce you the extremly example of lack on that department: AD Assassins. Zed and Talon are forced to build
and
, they can't simply throw away 1300G to buy an
without putting themselfes behind. I know very well that
on board's is a very popular item that is used as argument against AD mid laners - wich lmao, is a totally joke. Not only you delay by a lot your CORE items, but the upgrade of
wich is
is simply terrible on the assassins.
And if you think I'm wrong, the difference is very clear here:
is build almost by any mage, even when not facing assassins such as Zed for example. It's one of the most popular items on mages, meanwhile how many times do you see someone building
if not in cases where literally the enemy team is full of magic damage? Comparing
to
and
to
is ridiculous.