You can say the same about Rohit Sharma ,if he debuted in 90s he would have inferior stats to Navjot Sidhu but the fact is that it's agreed that a great player in any era would be great in any other era.
He would have a different technique and style of play if he debuted in the 90s.
Otherwise we could say that Jack Hobbs would have averaged 10 in this era had he played with that technique
Actually it’s not the case for batting. Sharma would have been a top Indian wicket bully even in 90s - reason, his basics of playing spin & medium pace is great, his defence is good. He could have a 20-25% discount on his stats, but still a very good player.
Problem for JB is that his defensive game is too poor, which is not been exposed for the playing conditions these days, which is allowing him to keep on batting for long, hence that average of 40+
I can name one player of 90s - Ramesh Kalu, the Lankan WK. He was a stunning stroke maker, blasted Aussie pacers in Australia; yet ended up with an average of 20, because in those playing conditions of 200-225 per scores, you’ll get tougher challenge (read wicket taking ball) in almost alternate ball. Butler could have maintained a very good SR even in 90s (not 120, for that you need to pull the boundary line in, no reverse swing, compressed heavy bats and machine stitched balls that hardly swings after few overs, doesn’t spin much either), but his average would have suffered a major hit.
At the same time, 25 years down the line with current playing conditions, Ramesh Kalu won’t have managed 120 SR (he doesn’t have the physical attributes for that), but he would have averaged significantly higher with a very good SR. Unless you consider the context, comparing ODI stats of different era will be misleading.
JB is a brilliant LO player for the era he is playing, which I do agree.