Whether more drugs will be used by more people depends in large part on how legalization is implemented.
If, say, heroin ads and a wide variety of colorful heroin preparations with outrageous claims are allowed when heroin is legalized, as they are with cannabis then yeah, it's likely that use could rise significantly.
But it's unlikely that the legal heroin market will be allowed to be the kind of free-for-all that cannabis has had. Ads will probably be forbidden, and there'll probably be just one or a handful of heroin preparations in white-labeled containers, as there are for generic prescription drugs... and you'll probably need a prescription to get it too... and you'll probably need to register as a heroin user and maybe even be required to undergo treatment.
So that's a totally different approach to legalization compared to cannabis. Heroin and cocaine, if they're ever legalized will be heavily regulated, and then not nearly as many people will be drawn to using them, compared to cannabis which most people think is harmless and which is advertised heavily wherever it's legal, with a million enticing preparations of all kinds and no prescription needed to get it.
Something else to consider is the example of the Netherlands, where it was shown that Dutch teens were much less likely to use cannabis (despite it being effectively legal in the Netherlands) than their American teen counterparts when cannabis was illegal in the US. The Dutch teens just didn't think cannabis was cool, unlike American kids who were drawn to the forbidden fruit.
So, paradoxically, drugs can be made to be less enticing when legalized than they are when they're illegal.
Yet another thing worth mentioning is that pretty much everyone who actually wants to use a given drug can freely do so now, while it's illegal. It's not unavailability but disinterest or fear of addiction that's keeping most non-users away right now. That won't change with legalization.