No, I don't think many more people would use heroin if it was legalized. You're putting words in my mouth and answering a question I never asked.
Legalizing drugs would make it impossible to keep heroin (and other drugs) and all its associated consequences out of public sight. I've lived in liberal cities like San Francisco and Seattle that have unofficial policies not to arrest people who have drugs up to a certain amount. That's how you end up with junkies openly smoking meth and visibly shooting up with needles in public, crazy people causing crazy people problems, skyrocketing petty drug addict crime like shoplifting and vehicle break-ins, etc.
All those things are virtually impossible for police to actually catch people in the 10 seconds it takes to do them and prosecute for. The way to prevent them from being something kids see everyday is to keep people on drugs away from the public. You keep people on drugs away from the public by arresting people who have drugs.
I used to walk by passed out naked people, people shooting up heroin and smoking meth, etc. everyday. Kids in those cities had to see that shit every day. Americans won't let that become the new normal in America.
Americans aren't going to turn the entire country into what San Francisco looks like to solve Mexico's problems.