Other items can't be countered by simply "paying attention". You just said, the item only works if your opponent isn't mindful. That's the definition of a bad item. It only works if your opponent is bad.
Try NOT making Zephyrs, and instead, making appropriate items for your carrys. See if you don't win more often doing that.
I feel Zephyr is what people make, because they don't want to learn good itemization.
You can indeed build to counter. You can see early in the match how it's going. You can assume, that you will want grievous wounds. Somebody gonna make Olaf. You can see very early if somebody is trying to build a Veigar. There's a few ways to counter that, and try to include it in your comp.
"Hush" counters Mages. Mages want to build mana quickly to double Ult. You can slow that down. What are they including with their mages? A Nami? Better get QS on some assassin to take her out. These are just examples. The fact that you think that Zephyr is good, and you also don't believe that counterbuilding is possible, isn't a coincidence.
if you make it to late end-game, then you will probably have 9 complete items. By that time, your opponent is certainly familiar with your Zephyr.
Anyway, this is what I see. Guys even make 2 or 3 of them. I build very large comps with many synergies. I'll have some units just for synergy, and otherwise quite weak. I'll most likely make sure that the enemy Zephyr targets those. Or else my champ with a QS.
We should check Zephyr winrate as an item...
