No, it may be a weed, but pot growers put an awful lot of effort into producing it, especially the pricey stuff. It’s still relatively cheap, and it will be a whole lot cheaper before taxes and excises when the risk of arrest/seizures goes away and they start mass producing it with modern agricultural methods. There will be favorite brands with brand loyalty. Consumers will demand better taste, aroma, less harsh smoke, etc. Homegrown crap that Joe Blow grows in his backyard will be far less appealing when consumers can go into a nice clean store and select from a wide variety of product. Homegrown product will take a portion of the market probably not much bigger than that for homegrown tobacco and home brewed beer.
As for decriminalization, that might save police and the courts some time and effort, but it won’t create legitimate jobs. It won’t bring us tax revenues. It won’t separate marijuana from the really bad stuff like meth and heroin so that marijuana smokers no longer buy their product from the people who sell these other drugs and so that smoking marijuana no longer makes people “fellow illegal drugs users” to those who use the hard stuff who today don’t really have to worry about breaking that stuff out in front of marijuana users because marijuana smokers are already fellow law breakers. And, it won’t be a major blow to organized crime like just regulating and taxing it would be. Marijuana is the backbone of the illegal drugs industry. More people use marijuana than all other illegal drugs combined, several times over. It generates billions and billions of dollars on the black market. It is the illegal drug most in demand and all others ride on it’s coat tails through the same smuggling routes, distribution networks, etc. The marijuana trade is where the sellers of the hard stuff recruit people to transport, sell, and purchase their far more harmful product. We need to take marijuana completely out of that realm. Take the billions from organized crime and create a legal industry with law abiding tax paying people taking all the jobs it will create. The benefits of outright legalization, regulating and taxing a legal marijuana industry, would be huge compared to the benefits of decriminalization, and if there is a downside to either the difference in that between decriminalization and legalization would be minuscule.