One thing people miss is the relative quality of teammates. Kohli has Rahane, a criminally underrated player; Pujara; Vijay and many others capable of consistently contributing to the team. Playing in a settled lineup makes a world of difference to any batsman. Williamson, Smith and Root don't have that privilege. They're all captains (except in the case of Root) of very young teams, ones which are still in transition and will not mature anytime soon. Only Smith has a teammate who's anywhere near him in terms of quality in Warner but this is negated but the rest of the lineup being completely unsettled/shit.
Remove Kohli and India would still continue to perform at at least 75% of their usual level thanks to the rest of the lineup. Removing Smith would only hasten Australia's path towards being the next West Indies and the same applies for Root/Williamson.
I am not posting to tell who is better than whom. I want to state only on the points which I have marked in bold.
T20 - If you have followed the world cup, then you SHOULD know that other than Kohli, no one performed well in the Indian team. He had so much pressure, he had tricky wickets and yet he performed very well. NONE of the other Indians played well in T20. Everyone from Rohit, Dhawan, Raina, Yuvraj and even MSD failed to perform. This is the case in all the T20 matches. Rahane is out of context here as he is not in the team in T20. Rohit performs well only in flat tracks. The only reliable batsmen ( apart from Kohli obviously ) we have got is MSD. But his T20I stats are poor.
ODI- Rahane is not a great player in ODI. He is a good player. But way behind Kohli's class. Rohit as I said performs mostly on flat tracks. And we don't have any other opener who is performing consistently. The only other player who is performing well in ODI is Dhoni. You can compare Kohli-Dhoni to Smith-Warner. Hell, Warner nowadays is more dangerous than Dhoni ( Dhoni's form has slightly dipped recently ). There is no other batsmen in our team whom we can rely on. Kohli can bat on any pitch and he will still score runs. But the same can't be said for anyone else in Indian team other than MSD.
Test- I agree partially with you regarding this. We have Vijay, Pujara and Rahane and dare I say Ashwin too. But, Rahane never plays well in India. Vijay is not playing well since past one year and Pujara started to perform well again since last series. And Kohli has the burdency of captainship in Tests. But again, we don't have a stable opener. I can say the same to England. They have Root, Cook, Stokes and Bairstow has been incredible for them recently. So, Root has comfortable team. Even for Aussies, you guys have Warner, Smith and Khawaja. You guys haven't had good series or 2, but Warner and Khawaja are great players. Especially Warner.
Without Kohli, India would be an ordinary team in limited overs format without any doubt. We rely on him more than we should. The guy is performing consistently since so many years and atleast in limited overs format, for me, he is the greatest. ( Apart from ABD ). Nowadays, he is performing well even in Tests. Kohli is a class act regardless. His performance and stats are extremely good in limited overs.
And I agree with you on Williamson. He is kind of a lone wolf in NZ. But I don't agree with you that Root and Smith doesn't have their teams support.
Well, opinion differs with each person, but that's what I feel.