@M3D
Yeah, but critical strikes scale faster than armor penetration.
You being like twice the health, with auto attacks dealing twice as much damage (due to higher crit rate + IE, you're getting like 50 AD from atmas)
at that end part of the game, cleaver wasn't helping you enough against their tanks, and you had to kite better to be safe due to your squishieness so you couldn't freely auto attack whoever you wanted anyway.
The main thing about late game ez that's "the strongest you can achieve" is:
1. Near 40% CDR
2. Arcane Shift hence is on the shortest of cool downs as if you were Kassadin on steroids
3. Mystic Shot being a very strong source of health sustain (Tforce proc + bloodthirster life steal).
4. The defensive amounts so you couldn't be bursted down instantly (which did happen to you and I as the squishiest on our team several times)
5. Slowing people (Tforce proc + red buff)
NOW after those 5 things you're looking at "maximising damage", Last Whisper would be awesome, but you're sacrificing awesomeness from triforce (realistically at late game you care about the slow and movement speed, not the sheenproc) or bloodthirster (your sustain in a fight / what let's ez 1v5 people), or defensive item of choice (Ezreal -can- be played without defensive items successfully but the level of positioning required is quite extensive / defensive items give ezreal a larger error margin) or boots (you'd have tforce but you're still slower than tier 2 boots) or infinity edge... well yeah, you can get rid of that and suddenly you have a strong core item while still doing critical strikes of good amounts and you can murder tanks a bit, however against the squishies on their team you are being outdamaged.
tl;dr Warmogs, Atmas, Bloodthirster, tforce, last whisper is strong enough. Switching out LW for IE would have made murdering vayne/cait/anivia? (I tthink it was anivia?) that ever so much easier by using auto attacks.
But you still would be a wrecking ball without IE. And would have dealt a lot more damage than the build you had at the end game (the same build you had for like half an hour)