You're completely correct in saying that Pantheon falls off lategame.
But it's not some kind of weird glitch in the game that makes Pantheon super weak, it's just maths.
So if you can work around the reason why Pantheon falls off, you can actually stay super relevant.
If you look at my build that game you see deathfire touch and a lord dominik's regards.
This means that on top of my flat armor pen items, i do a shitton of %pen
Rammus and Taric can only CC some targets so you just wait until they don't have CC to interrupt your e anymore, so you jump one of their carries,oneshot them with full combo and probably if lucky you do an insane e %armor shred on their entire team