pant and poor scaleing? faling of lategame is not that much a matter of scaleing numbers, but off how the kit works.
Pant has massive scaleings, his Q+E is on incredible low cd and has a higher ad scaleing than Lees entire combo with ult.
The problem is, pants E is a short range channle, and pant has a noticable cd on his mobillity, so in a 5v5 fight(which is rather common in the lategame) he has problems at reaching the right targets with his damage, even though the damage potential is really high. He also has not that great base and little cc, so he can't really go tanky, so either he can do high damage, but most likely gets cced and dies befor he can really use it, or he does not that great damage but can atleast survive a bit.
Same with lee, if he doesn't get a great flank, he will have problems reaching the right target with his Q, and most of his damage is singetarget.
He also has huge energie issuses if he can't use the Passive, and in a teamfight vs some cc, it becomes quite hard to land some AAs in between you skills as Lee.
Also, Talon does fall of pretty hard in the late-ate game. As soon as his target is able to survive his burst, he drops hard. He has no way to escape after his first burst, nor to follow up on it.
And besides that, he still is better latergame than pant or Lee, as he can perform his entire burst within a sec after a instant gapcloser, which makes it pretty impossible to prevent him from bringing in that damage, and aditionally most of that damage hits on a big aoe.