Thank you, Goombah!
McRod, Goombah gave you a great explanation of why Sollozzo thought his scheme would be advanced by whacking Vito. That's it in a nutshell: the judges would be up for grabs. They weren't so much loyal to Vito as to Vito's money and influence. With Vito gone, they'd still have to eat, send kids to college, etc. At least some of them would, in effect, sign on with Sonny. And since Sonny was "hot" for the deal, Sollozzo thought he had a good chance to get his way with Sonny after dispatching Vito.
I'd add that Sollozzo also probably was a "climber. According to the novel, he was from Sicily, without a family in NYC. By whacking Vito and imposing his will on the Corleones, he'd achieve instant status among the surviving Dons--perhaps even as an equal.
As for Luca being whacked so easily: the short answer is that it makes for a great dramatic scene. Few viewers of GF will ever forget the knife in the hand, the bulging eyes, etc. In real life, Tattaglia and Sollozzo probably would have used at least two gunman.
Your post raises another question: If Vito hadn't been "slippin'", could he have stopped the drug traffic? I think he could have--temporarily. If he'd been on his toes, he would have seen that Sollozzo would never take no for an answer, and that he'd be a mortal threat. He could have given Sollozzo a tentative "yes," and arranged for him to meet with Luca to work out details. Luca could have introduced Sollozzo to a drug dealer of his acquaintance. Then, Luca would have murdered both of them to make it look (to the police) like a drug deal gone bad. The other Dons would know immediately that Vito was behind it. But it'd be like a shot fired across their bows: No drug dealing while Vito Corleone is top dog in NYC. Tattaglia, the pimp, would have to suck it up. Barzini would realize that, with Sollozzo dead, he'd have to line up another major global drug trafficker before planning a move against Vito. I'm guessing that this "pax Corleonus" would have lasted until Vito died. Then greed would have taken over.
