Don't be fooled by KW's average as that query is very general.
Here's a much more telling stat for me.
If we can all agree that since 2000 the best 5 tests sides have been Australia, England, South Africa, India & Pakistan (in no order), this is where Root & Smith really show their worth, with Kohli not far behind.
In fact Root & Smith have the best records against top quality opposition of all batsmen since 2000s, both averaging over 55. Meanwhile, against top opposition Kohli averages 51 & KW way down below 40.
In KW's case, he generally relies on murdering relatively weak Windies and SL attacks, and he did admittedly do well on some roads in Australia in 2015.(Not so well in the more difficult deck in the Adelaide Test though).
I do think this is a fair indicator that at least against the best opposition out there, Smith, Root & Kohli (not far behind) are well ahead of KW, who only comes closer once the Windies, SL, NZ & the minnows are brought into the equation. Hell even Ross Taylor has done better against the top sides than KW if we're honest about it.
Best average with min 2000 runs vs. Eng/Aus/Pak/SA/India since the year 2000