Think of it this way. I have a vegetable garden, banana trees, mango trees, and lemon.
Yet, I still purchase these items from the grocery store. In order for my personal demand to be met I would need much more of my real estate space to be occupied by these plants. It's just not worth losing more real estate space to grow more. This does not include the time I need to spend nurturing/tending to these plants. My time is more valuable than I'm willing to put into more plants.
The space and time you need to grow your own MJ product is more demanding from what I have read. It's not say there are people out there who are really into it and are willing to pay such demands, not unlike creating your own brew. I don't fear growers being able to grow their own product, there will always be demand for regulated, easy to obtain product. Look at Canada right now. People can still grow their own yet all these LP's are showing up and obviously selling pretty steadily.