If khabib defended his belts 3-4 times, He'd be everyone's Goat but he chose to retired because there's nothing left for him in that division.
Khabib certainly hasn't cleaned out the lightweight division. Charles Oliveira is on 7-fight winning streak, all by finish. Khabib was scheduled to fight Tony 5 times, but never did.
Khabib also enters the cage heavier than some welterweights. If he wants to prove that he's the best pound-for-pound fighter, he should be fighting in his natural weight class. It's not lightweight, as evidenced by the fact that he missed weight twice in his UFC career and needed a towel for his most recent weigh-in. He should fight his peers at welterweight. The top tier of that division is filled with elite fighters: Usman, Colby, Burns, the winner of Chimaev-Edwards, and Lima if the UFC can finally bring him over.
Of course, the legacy fight with GSP is still out there. Khabib better get that done while GSP is 39 years old (he has 7 months left) if he wants to solidify his claim to GOAT status.
He only defended the lightweight title 3 times. There are plenty of fights for him. He didn't clean out anything.