What do you consider "falling off"?

So I'm calculating how to maximize Yasuo's maximum DPS (Note: Not all my notes are present here due to how I delete most of my notes when I'm done with them) and I noticed part way through that after Yasuo builds his Infinity Edge and Statikk Shiv, no other 5th item slot item comes close to the gold efficiency offered by 4th item slot Shiv (at least for extremely low levels of Bonus Armor as far as Black Cleaver and Lord Dominik's Regards are concerned).

This problem is most evident in Yasuo due to some really easy to hit hard limits in his kit (you don't need that much attack speed to get his 1.33 second cooldown on his Q, and with the Critical Chance items, 100% crit chance is really just too easy), but I should note this sort of issue can show up in any champion, especially AD DPS champions. The reason why you build multiple damage items for most AD DPS champs is because all the stats you build multiply into each other, giving you exponential growth as you build more and more. So, vice versa, wouldn't that mean if you're not noticing that exponential growth as you build more items, doesn't that kinda defeat the purpose of building more damage items?

Just wanna see what others think. The fall off is dramatic and I can make a graph if anyone wants to see what I'm saying in a picture, but the point is building more than aboutish 4 damage items on Yasuo sees dramatic fall off. At that point, should you stop building raw damage and start focusing on tank stats and utility?