When exactly has Smith ever been 'average'? His first few Tests he was mugged off and played as a leg spinner. His batting record throughout all levels has been fantastic. Whether he maintains his average or not, he'll average 50+ when he's retires.
And I don't see Root being any class apart from Smith.
He was massively average when messer flintoff took him apart. He was dropped and went back to shield cricket. He has changed his technique and since coming back has done very well Except when the ball is moving, then he looks avg again (same as most modern players). To sum up, again the back drop of modern players who are in the most part white ball players he is one of the best. Is he a patch on some players of yesteryear ?? No in my opinion but the game is more white ball orientated even in tests than before so tbh, you can't compare modern players with your pontings, heydens, langers, martyn etc.
Root is a different player to smith, totally different style. Root has just as many weaknesses as smith (outside off stump early on which as he keeps being touted as a top order bat is horrendous).
Both are modern batsmen so let's just judge them against those around now. Avg 50+ now is not comparable to a ponting/Lara/kallis etc