You mistake the idea of an early game champion. An Early Game champion means they have a gameplay pattern that makes them best suited for laning scenarios. Their primary strengths are in situations where they don't need to think about anything but killing the folk right in front of them. Where they suffer, is their ability to contribute to teamfights, or traverse teamfights.
They never get any worse at their ability to kill, they're simply put in to situations late game where their strengths early game are no longer relevant, like Renek's ability to Dash-Stun-Damage-Dash out, with little reprecussion to himself. This same strength isn't found late game, where ideally, everyone should be grouping up as a team. Renek, if he got ahead, should still have kill potential all the way to late game, but that doesn't make him a sitting duck once he gets there, if he's still ahead of the other champions.
Same as Pantheon, he never loses his ability to kill people, he simply loses his strengths to poke you in lane with his spear, and his dedicated combo-stun. Still just as likely to kill late game, but he's not going to have those one-sided strengths like in laning. Same as Illaoi, her having her tentacles all around her in lane, but less likely when moving with a team, but she can still kill you regardless.