That "Sheer amount of True damage" exists in many forms: Vayne, Cho gath, Camille's Precision protocol converts up to 100% of her damage to true damage.
Fiora does up to about 20% of a target's HP in true damage per Vital she hits.
Gangplank's Passive does up to 225 + 100% of his AD in true damage each time he destroys a barrel and auto attacks after.
There's a lot of sources of LARGE true damage already in the game. I didn't Mention Yi or Irelia, because theirs is moderate.
The true damage on Zoe's E is actually around rank 8 in True damage hits. It Caps at the damage (base +AP) on E's own damage, which means that even at 1000 AP, its cap is 220 + 400 = 620 True damage added to whatever attack procs it.
I want you to look at Darius's Ult or Cho's Ult.
Darius's ult when hitting bleeding stacked foes on 5 charges of bleeding can do around 1300 -1500 True damage (This is why a snowballed Darius can Pentakill and has very little counterplay)
Cho Gath's Ult scales infinitely, and deals 650 + 50% HP + 10% of his Max HP (aka a 4000 HP cho deals at least 1050 true damage, and if he has AP, more)
Fiora's ult allows her to hit up to 4 vitals, all of which on a well built Fiora = at least 80% Max HP TRUE DAMAGE In addition to her AD on hit damage, and a self heal (She's the #1 sure thing true damager)
Vel Koz's entire Ult does True damage if the target takes 3 stacks of deconstruction prior to the Ult (That's potentially a 2000 aoe True damage hit)
Furthermore, Zoe's effect is unreliable. If the next damage source is just an auto attack, many times the damage doubled is a pithy amount and it's wasted.
And, range is also unreliable. If you shoot from too far as Zoe, you literally wasted your bubble entirely. You need to be within 1375 range at minimum to actually hit anything with any combo of skills. Most wall jumping shots put Zoe beyond that, and the MOST reliable 1 shot combo requires you to be at 800 range (and R backwards, proc Q, and then reproc after R ends). If you attempt 1375 range R over a wall and then Second proc Q forward, 90% of the time it will miss. At high range, Bubble is pretty much a support spell for a teammate.