Well, they would certainly have reason for wanting the killer off the streets, as I'm sure the heightened police presence and vigilante groups weren't doing wonders for business. That's why I believe that the Lusk letter was sent by some local crook(s) to intimidate Lusk because the vigilance committee were stepping on one too many toes. As for the criminal underworld getting to the Ripper, I think they had as much chance as the police, to be honest.
Back in the 1970s a series of novels about Professor Moriarty were written, the first being "The Return of Moriarty". In that first novel, he is privately reminiscing about the "autumn of terror", and how he is put on the trail of the Ripper by one of the victims (before her death). He and Moran catch the Ripper, and make his death look like a suicide. The perpetrator's motive is to bring the conditions of the East End, especially prostitution, to the public's notice. The novel was written soon after the publication of Tom Cullen's "Autumn of Terror", so the novelist was using Cullen's suggestion of a Ripper motive. Moriarty's involvement was that his own extensive prostitution empire was jeopardized by the Ripper's activities.
Do you find the following reason for the end of the murders plausible:
Jack was caught by the London Underworld and vanished because of that?
All the best
The Jewish underworld !
On my travels I met an elderly Jewish man whose relatives lived in Whitechapel at the time of the murders. He told a story that prominent members of the community identified the killer as being a Jewish man.