Technique is not dull. Technique is very important. It a proven fact throughout test history that only good batsmen with excellent/very good/good techniques have forged excellent careers vs good pace bowling attacks - especially openers. They are very few players that have defied this trend in 136 years of test cricket.
No team with batsmen full of Sehwag like techniques has ever won any test match or series in history, that is for certain.
For all those quick double hundreds Sehwag has scored & did all that you mentioned, he never once did it vs a quality bowling attack on a helpul pitch. All were on roads. Roy Fredericks superb hundred @ Perth in 1976 is the only instance in test history where an opener blasted quality pace bowling attack with such fury on a helpul pitch, that Sehwag played some on his big innings on roads.
This shows clearly how hard & nearly impossible for openers or batsmen in general to be so dominant vs good pace/general bowling attacks in difficult/testing batting conditions.
All of Sehwag's shot gun innings were on roads vs poor attacks. While inversely he has ignominy of having series vs good bowling units where he averaged sub 20 on many occasions. None of the
great openers in test history have such a glaring black mark on their records.
This galling trend of his career shows him to nothing more than a FTB. Where were some of those innings in recent times when IND toured ENG, SA, AUS?
The fact that he scored such runs so dominantly more than other batsmen in the 2000s era, which was the worst era of quality fast-bowling & flat pitches since the 1900-1939 era, is not a reason to qualify him as great. That is theory seriously lacking sane cricket logic.
In the 90s for example you only had Lara, Tendulkar, Steve Waugh & Graham Gooch passing the benchmark average of 50 which separated the great batsmen from the rest. In the 2000s, Sehwag was among one of many batsmen (almost 20) who hit a purple patch whose average skyrocketed to & past 50.
Dominating bowlers in during the horror days 2000s era is nothing to write home about for any batsmen. Their records given the paucity of bowling talent they faced needs to taken with greatest pinch of salt & weighed in proper context. Especially when wants to consider them a great batsman in comparison to other batsmen of previous era's who faced top fast bowlers almost every series.
This where i feel batsmen like Mike Atherton & Ian Redpath & Colin McDonald would always be under-appreciated in modern times, but thats a debate for another time.
Considering all these factors & facts to consider Sehwag as an opener in a cricket All-time test XI & one of the greatest openers ever is venturing into the realms of the unhinged.
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You both make very good points and I'm not inclined to side either way. However, I will say that India probably hasn't had a game-changer like him, except perhaps Kapil Dev. Flat pitches or not, why were other batsmen not able to do the same as him? I accept that he is poor outside the subcontinent and that this is due to his poor technique. However, in the subcontinent, he has been virutally peerless, not in terms of the amount of runs he has scored, or his average, but how he has single-handedly, not shaped (e.g. Dravid, Zaheer, Tendulkar), but changed matches. For quite a few years, he was a counter-attacking batsman of the highest caliber.
However, it is important to keep things in perspective and realize that he hasn't been performing even in the subcontinent for a while now and in my opinion, no longer merits selection.
Sehwag career as a batsmen is very similar to Kumble as a bowler before the 2003/04 tour to Australia.
Kumble was a beast at home when India were invincible in the 90s, but away from home for more than a decade he was at best average.
In the last 4 years of his career after that AUS series, Kumble became equally good home & away and many then recognized him as a all-conditions spinner.
Sehwag has failed to make than transition as all-conditions batsmen, which a key pre-requisite to being considered a "great". Plus as you correctly said, he hasn't even been able to be dominant in the sub-continent even vs the good bowling teams either in recent years. The improvement in pace bowlers in recent years have even figured out how to keep quiet at home.
That is an area he should still be still be good at, but the fact that he hasn't is another indictment on his career, which is coming to the end of its tether.