"Fall off" does not apply only to damage. Fall Off can apply to simply being in situations where it's far harder to do your job. For example, in lane, Renekton and Pantheon have high pressure, and easy trades that limit their own damage. However, come late game, Pantheon's single target nature and having to deal with more than one person results in him falling off, because his trade pattern is far more dangerous to himself. He circumvents this by transitioning to an engager/splitpusher, using his ult to influence and pressure the map. Pantheon is still by far able to kill people, but it's considerably harder too because the way he plays makes it harder.
In my opinion, which you may disagree with, Lee Sin falls off because his strengths in dueling and ganking are far poorer late game. Not because he doesn't do enough damage, but because when teams are grouped, his target selection is extremely limited. Unless he outplays by managing to flank from behind, flash a Q on a mispositioned carry, or be fed enough where it doesn't matter who he murders, Lee Sin can't engage on squishier targets, and while he's not bad with killing tankier targets, they're not going to die fast enough.
Now, he has a plethora of tools to circumvent that somewhat, like his ult doing more damage based on a target's max Hp%, making a tanky frontliner into a potential dangerous projectile, but that alone isn't enough to circumvent the weakness that Lee is good at dueling, not handling multiple targets. It's very similar to assassin's in that regard, where they're great at killing, but struggle in a teamfight were a tank/support could interupt them and save their carry.
Once again, it's not that they don't do enough damage come late, it's that their strengths are geared better towards early, and they require a substantial amount more effort to affect the game. Pantheon needs his ult to get to squishier carries, Renekton can't dash-stun-damage-dash for free damage, Lee needs to rely far more on his tools to, Juggernauts struggle with being kited in teams, Assassin's can't do their job easily with supports and tanks, etc, etc.