I don't know what should surprise me more: Herbert Asbury's assertion in his Introduction to his book The Gangs of New York, first published in 1928, that "there are now no gangs in New York" (Thunder's Mouth Press edition, no date given, but with a copyright 1998 translation of a text by Borges serving as a Foreword, and a blurb on the front cover claiming that Scorsese' 2002 movie is based on Asbury's book, p xiii), or that I can't find anyone who has described this assertion as astonishingly ignorant, asinine and so forth.
Once again, I must do everything myself. Asbury's assertion is astonishingly ignorant. It would be astonishingly ignorant if it had been said by any American in 1928, let alone someone like Asbury who had written an entire book about organized crime in NYC. No gangs in New York in 1928? That's an incredibly asinine thing to say.
No gangs in New York City in 1928? Who, exactly, did Asbury think had furnished the Prohibition liquor he was drunk off his ass on when he wrote that whopper? He concedes (ibid, p xiv) that there are, in 1928, entities known as mobs, but he claims that gangs and mobs are two very different things. I've never heard anyone else claim that gangs and mobs are two different things. Asbury says that gangs had relied on the co-operation of corrupt politicians (p xiv), and seems to imply that political corruption, like gangs, are now, in 1928, a thing of the past.
(I refer the reader to any written account at all of Jimmy Walker, Mayor of New York in 1928. Walker does not appear in the index of The Gangs of New York. But Asbury does mention Tammany Hall -- a sort of huge factory which produced political corruption on an extremely efficient basis in NYC from no later than the 1790's until at least the 1960's, for whom Walker worked -- several times in his account of things which he says disappeared by 1928. It's not clear whether he realizes that Tammany Hall was still in operation in 1928.)
He says (pp xiv-xv) that mobs seldom consist of more than 6 or 8 members, that they are temporarily formed for a series of "robberies or other crimes," that they have no special allegiance to their leaders, and that they don't have rivalries with other gangs or fight with them over turf.
Martin Short, not the guy who's famous for saying hilarious things on purpose, but a British author described on Wikipedia as "best known for his exposés on the Mafia and on Freemasonry," notes that Asbury's assertion that gangs were absent from NYC in 1928, and that mobs were not gangs, is in error, but still says, in his modestly-entitled The Rise of the Mafia: The Definitive Story of Organized Crime, that Asbury is an "excellent [...] popular historian."
It gets sillier: In the New York Times in 1998, a columnist named Joe Sharkey wrote that Asbury was right about the absence of gangs in New York in 1928 -- more precisely, he quotes Asbury to that effect and gives no sign that anyone shouldn't take Asbury's word for everything -- and goes him one better by saying that the Mafia appeared after 1928, and now, in 1998, was completely gone.
Sharkey is just a prominent case, one case of many, of people taking Asbury's word for it. Short is one case of many of people not calling Asbury an idiot for saying there were no gangs in New York in 1928.
Perhaps in 1928 Asbury had spoken with J Edgar Hoover, and Hoover had told him that mobs were not gangs and that gangs no longer existed in New York, and Asbury had taken Hoover's word for it. Hoover said many times over the course of several decades that the Mafia didn't exist, and a lot of people took Hoover's word for a lot of things they shouldn't have.
Perhaps there are reasons why people can take Asbury's word about some things. I don't know.