Kohli’s rise and decline were both spectacular. He averaged over 55 at the 81 Test mark - that’s a pretty awesome performance by any means. His decline was so spectacular that his average came down to about 47!! In his first 70-75% of his career, he performed like a champion. He created an awe around him. Commentators from all countries spoke very glowingly about him. From an overall career perspective he ended up being just “very good”, but in his peak years he was churning out ATG performances.
People have beaten him up for the big slump in the last 5 years of his Test career, and rightly so, but nobody can take away what he did in the years he was shining bright.
He had an aura around him. He brought eyeballs to the game from non-Indians too, people wanted to watch him, he brought passion, fan following (often unwanted craziness from fans), and equally importantly, sponsors to finance cricket. He made people opinionated - there was nobody on the fence; either you liked Kohli or you hated Kohli. His own opinion mattered; people caught on to his words to make Test cricket relevant.
In my personal opinion, Kohli ended as a very good Test batter overall, but his impact on Test cricket was absolutely ATG level without a doubt. With Test cricket losing appeal, he was one player everyone looked up to for keeping it relevant and he greatly led that battle to keep the flame burning and inspiring the next generation.