They would not, lol. Brewing one's own beer has been steadily rising in popularity, but do you see people selling it cheaper than Coors? Hell no, and generally it is much, much more expensive, and still uncommon to boot because it takes a lot of time, effort, knowledge, and money. Same with tobacco- do you know anyone who grows their own? I don't, and have never even really heard of it. Big corporations are always going to be able to streamline a process for mass production and sell cheaper than small-time manufacturers, much less someone growing in their backyard.
There's a big difference in brewing your own beer and tobacco It costs money to do both. Wanna grow your own tobacco? You're going to have to have the land to plant it, good soil, the correct season, the time, and the hard work of picking it, planting it, plowing it, then drying it out and everything else that goes along with preparing it either for smoking, dipping, or chewing.
As far as alcohol goes you're going to need all of that and more and it's simply not worth doing unless you are some kind of moon shiner or just really want to drink you're own alcohol.
Which is exactly what you said, i'm just using those points to contrast.
Weed is much different. It's easier to manage and grow and it's simple to pick. There's no processing, fermenting, and much less know how required to do it. If weed was legal 80% of the users would simply grow their own weed or buy it under market value from one of their friends that had a surplus.
Again, not saying anything is wrong with weed other than it being illegal. I'm only saying it being legalized does not mean that people will stop growing it/selling it. Surely not on the same scale as they do now.. but it will not be like tobacco nor beer in the same sense.