Once Khans retire, so will the word superstardom..!!
All three jokers you have named does not have it in them to hold the audience in the palm of their hands like khans did, no matter hit or flop, blockbuster or disasters movies are, their craze and madness is still intact..!!
In future Stars alone won't pull crowd, overall packaging of movies + star will pull crowd, you'll have to show people that your movie have what it takes to spend hard earned money unlike khans for whome name is enough, yep, genre might fluctuate numbers up and down in initials but still magnitude is humongous, a songless, thriller movie which opened to 19 crs, replace the lead actor and see the result..!!
Plus three jokers you mentioned are also charisma vaccume, barring few good stuffs here and there, mostly these guys have big talent of boring, irritating and making people feel the need of saridon & disprin..!!
In short, No one is replacing Khans, and once they retire, Bollywood will lose it's charm and once you lose your charm, you become dull, so retirement of Khans will make Bollywood in what we say in simple language............. DULL..!!