Yes but you had flat wickets in the 80s and 90s as well, and the modern batsmen would have cashed on it. Contrary to popular belief, majority of the so-called legendary batsmen cashed in on flat pitches as well, and bowlers have always made hay on grassy pitches.
Someone like Warner exemplifies what a flat track bully is. He is rubbish against swing, seam and spin, but he can bash any bowling attack 9/10 on flat wickets. Now if you take Warner to the 80s and 90s, I don't think he will be reduced to a tail-ender: he will bash bowlers on flat pitches and he will continue to fail in difficult conditions. I don't see why he would not have taken the likes of Wasim, Ambrose, Hadlee etc. to the cleaners on dead wickets. Same goes for other flat track bullies like Guptill etc.
A lot of mediocre batsmen were successful in those eras on flat pitches and against the bowlers who are now considered to be one of the finest of all time.
Nonetheless, every batsmen has a weakness. Lara is regularly championed as one of the best players of spin, but he was not good enough to handle the Indian turners, and neither was Ponting. Kohli failed in England in 2014, but he will get more opportunities to rectify his record there, and even if he doesn't, his tremendous record in Australia, SA as well as an Asia is enough to put him on his way to legendary status.
There are no reasons to consider Kohli an inferior batsmen to Lara, Ponting, Tendulkar etc., especially when you consider what he has achieved at the age of 28.