Lee Sin only "falls off" if you build him full tank.
His AD scaling on Q, E, and R are really high, but they all scale specifically to** BONUS AD**. So you have to build damage to do damage. Since Lee has such good synergy with lifesteal, I think the optimal solo lane build is:

With this build, he gets quite tanky, but also does a shitload of damage, and he vamps up incredibly hard. So, when people talk about Lee falling off, it's only because they did the standard LCS meta, utility tank Lee build:

Sure, he'll get tanky, and the wards are nice, but his damage will be garbage tier.