I think the arrest of "El Chapo" Guzman dealt a psychological blow to the drug warlords operating in Mexico. Among them, he was the most powerful and considered to be untouchable. His arrest and imprisonment sends the rest of them a sobering message. Their days are numbered, and they should be on the run.
The arrest of "el Chapo" was a major accomplishment for the Mexican government for several reasons, some of them include disruption of the chain of command in the cartel, lack of leadership in the cartel business and lost of faith in the trade. There are many psychology and practical reasons for going after the drug cartel leadership studies have shown.
The arrest of "el Chapo" is more symbolic than actually effective. In fact, his arrest illustrates what a show the entire "war on drugs" really is. Instead of concentrating on addressing the problem, they chose to go after a figurehead who probably was no longer involved in the day to day narco business.
While arresting one very big drug lord is a big accomplishment, there are still plenty of people who will take his place. People are addicted to drugs and getting rid of the suppliers is only a temporary solution. People have to be taught that drugs bring bad things with them.
I do not believe that just because the Mexican government was able to arrest the famous drug lord "el Chapo" that Mexico is winning the drug war. There is still a lot of people out there that will be willing to take his place. And there are also a lot more people in the drug problem than just one drug lord.