This is most commonly noted between finite interactions, specifically Burst, Tankiness, and DPS in that order.
If you directly compare a Burst Champion to the EHP of a Tank or Juggernaut Fighter, the Burst champion usually can not kill the Tank or Juggernaut, especially within a single rotation. Early game, maybe if you have an early leverage, you can beat a Tank as a Burst champion simply by hitting them hard enough. Eventually though, the Tankiness of your target is going to be waaaaaay too much for you to realistically take down easily.
Similarly, Tanks can easily outlast a DPS champion in the early to midgame. Tank stats are cheap and easy to build up by the midgame, and at this stage of the game, most late game hyper carries aren't at their true powerspikes yet. However, as time passes, the mostly additive bonuses of defense falls behind to the mostly exponential bonuses of DPS.
Lucian falls off relative to other ADCs because he does not shred tanks well late game. If you pit a late game Lucian vs a late game Tristana or Vayne, maybe you can duel them if you play well enough. However, if you are comparing a late game Vayne or Tristana to Lucian when it comes to fighting a Tank, you'll find that the Hypercarries are going to melt through that tank way easier than Lucian possibly could once he burns through his initial rotation. He sacrifices a lot of raw DPS for Burst-like qualities, including some of Burst's weakness to killing Tanks. This is why Lucian falls off.
Remember, there are 9 other players in the game. Nothing is absolute, everything is relative. Lucian doesn't literally lose damage as the game goes on, it's just that his opposition over time gathers resistance to his particular strengths, or anything you can compare him to starts to lap circles around him in certain jobs.