I think he was shaking them down because that’s how you generate income you need to reward your men, and how you intimidate people into not saying anything when the muggers and pickpockets who deter foot traffic on your block aren’t getting arrested because they’re paying their protection money to the man in charge.
That’s a big leap to make. Why assume crime went down because Daemon recruited all the criminals into a syndicate? It’s pretty risky for the king’s brother to try a scheme like that.
Why not apply Occham’s Razor & assume crime went down because the criminals were tired of getting whooped on and decided to vamoose for Gulltown, Lannisport, or Oldtown, any place with less “energetic” policing?
Because Occam’s Razor doesn’t explain how Daemon knew Mysaria and Blood & Cheese. Or why he was called Lord Flea Bottom and Prince of the City.
As for “risky” – have you met Daemon Targaryen? This is the guy who spent the day after Viserys’ son died partying and singing songs about “heir for a day,” who gave his mistress a dragon’s egg, who launched a war of aggression and declared himself a king, who hated his wife and when said wife died in a “hawking accident” tried to take her family’s lands, who as a man of 30 seduced his 15 year old niece, who married Laena Velaryon without his brother’s consent, and then when she died, suddenly got hitched to said niece when he had her husband (his brother-in-law) whacked.
Daemon Targaryen is the acme of a Byronic figure: mad, bad, and dangerous to know. Honestly, Occam’s Razor cuts my way, not yours.