It sucks that we never got to see Khabib vs. Tony, but I don't think that fight would've been competitive. And I've been saying it long before Justin Gaethje beat Tony. I don't think anyone in the lightweight division can defeat Khabib, for different reasons but the main reason being that a lot of people, and even a lot of fighters have a grave misunderstanding of what Khabib's fighting skills actually entail. Sambo is not wrestling or judo or BJJ. The problems presented by an extremely high-level Samboist are ones that most MMA fighters aren't used to dealing solving. It's why Dustin Poirier said striking against Khabib was very difficult.
It's not just the grappling threat, it's how Khabib moves, strikes and how the purpose behind his striking differs compared to the purpose of a kickboxer, boxer, karateka, wrestler's etc. striking. Combine that with his genius grappling and you're playing checkers against someone who plays chess. I think Khabib retires undefeated. And if he loses, it'll be due to him fighting past his prime like Anderson Silva.