for those who want it legalised, how would you then regulate it, would you keep it as it is, with gangs growing/selling it, and private individuals growing it, or make it more similar to tobacco, with the whole thing done in farms for profit and such?
You are confusing decriminalization with legalization.
Decriminalization is merely making it so possession of small quantities is a civil infraction comparable to a parking ticket. The inherent flaw with decriminalization is it does not do anything to address the already established black market distribution. Due to international treaty this is the best that can be done, and a necessary first step that may eventually lead to a softening of attitudes and viewpoints and ultimately to legalization. decriminalization is a flawed concept it does nothing to deal with the elephant in the room (black market), and is only minimally effective in a very small scope, but it at least does not ruin countless live for mere possession..
Legalization is establishing control over the entire market, from production to distribution, it is taking the black market out of the equation, much like the current situation for for alcohol.
It is establishing control, Controls such as NOT having it sold in schools or to school kids, not smuggled across our borders, not grown in our national forests, and not having distribution rights determined as a result of gang violence and which ruthless criminals can kill their way to the top (Q. when is the last time you saw commonplace drive by shootings over alcohol distribution?).
In order to establish control over the market, we need to assume control from beginning to end in the manufacture/distribution process (and no this does mean government run marijuana farms - look at alcohol distribution, it is controlled, yet companies such as Jack Daniels and Sam Adams are involved, not gangs and NOT our government).
One follow up question/objection is often asked, and yuo already mention it in your question, that is what do we do about those who want to just grow their own at home?
The answer to that is to allow it, but only with a permit, and appropriate tax paid (reasonable cost and readily available, unlike the Harrison stamp tax of yesteryear). If someone is growing privately without a permit, then they are guilty of tax evasion, and can face the penalties for that (this is what those running illegal shine stills get busted for.. off the top of my head I think it carries 10 yrs/$10k).
Since a major reason for legalization is to eliminate distribution/availability to children, there also must be safeguards to make sure a minor cannot just stroll onto a pot field or a home garden and pluck a bud at will. So outdoor gardens will be costly, and will inevitably move almost all of the private grows indoors, or to greenhouses.
Private growers cannot sell to third parties, and all products from sanctioned distributors are to be sold in a controlled environment much like alcohol is currently.
We have a model in place already, we saw the positive effects of ending prohibition of alcohol and reestablishing control, yet when we see the same problems inherent in a black market that were there during prohibition, we continue to pursue policy which created this black market, and created the criminals, the cartels, and the gangs in the first place. We have failed to learn from our past mistakes and are repeating failed policy.
almost 40 years into this "War on Drugs", and drugs are cheaper, stronger, and easier to get than when this all began, so too is violence and crimes committed as a result of the black market. Prohibition is a failed policy, and we are in a failed war that has had an astronomical cost both monetarily, and socially.
As a result of prohibition, not only do we still have a drug problem as bad or worse than we began, but we have also compounded it by creating a monster of criminal problem as well.