Newsflash: Ezreal isn't broken
They fixed the one problem that he was having. Now he is no longer proccing muramana 2 times in the late game.
Klepto doesn't in anyway make Ezreal more broken than any other ranged champion that takes it. Klepto is just broken on ranged champions. It isn't just Ezreal.
I mean I would wager that the problem is the boards is just bad at positioning to dodge his Q. It happens every time an on hit item comes out strong. First thing to blame is Ezreal, but half the time I play him people don't even try to dodge my Q or even make it hard for me to hit them with it. Even when they do it seems like they also forget that minions die so I can snipe them with a Q right as a minion dies. And to top it off his late game is completely reliant on hitting Q's which is the point that you should have the most MS to invalidate almost half his damage and your CC is up more often.
I think the only time Ezreal was a problem was when he was super abusing
as it covered his one weakness late game. It made his Q's easier to hit against more mobile teams. Klepto makes Ezreal sacrifice damage early and late for gold gain, utility, and some RNG damage early on.
I think another part is literally it was one of the first things to come up as good in the new season. I play a bunch of other champs and have found some pretty disgusting runes, but everyone is for some reason just trying to copy other people with a good rune page instead of finding their own. Because I can guarantee you if you nerf Ezreal for Klepto, that there's at least 5 other champs waiting to take that spotlight as stupid broken with the new runes.