It's a spiritual disease. You are correct in speculating that one addiction can be switched with another - cannabis, pornography/hookers/affairs, heavier drugs. The bad part is, unless one surrenders, if a person is a bona fide addict, their full blown illness will drive them to depression, unmanageability, prison... possibly suicide.
The good news is, if one surrenders, he or she can suspend the progress of the disease.
I really don't think a cannabis addiction can be classed as the same life ruining addiction that alcohol is. Maybe because of it's legality it can ruin lives but the parent was specifically talking about after it became legalized.
The following is a troublesome statement to me about pot that I didn't know until now:
"Smoking marijuana is more dangerous than smoking cigarettes, experts say. The tar in joints contains a much higher concentration of the chemicals linked to lung cancer compared with tobacco tar. And smoking marijuana deposits four times more tar in the lungs than smoking an equivalent amount of tobacco, according to the National Institute on Drug Abuse."
It's important to know that there are alternative ways to consume cannabis. One is to ingest it (which may not be as convenient as smoking), an other one is vaporization. Vaporization offers a very similar experience to smoking without the health risks associated with combustion (it's still not known if it's totally safe though).
One thing we can expect with legalization is that consumers will be better informed about those alternatives.
That's a clever bit of scaremongering, if you do the math. Marijuana smoke is more dangerous than an equivalent amount of tobacco smoke, but a typical marijuana user consumes far less than than the typical cigarette smoker. Nobody (or almost nobody) has a pack-a-day weed habit.
The good news is, if one surrenders, he or she can suspend the progress of the disease.