"In this case, cannabis will be like alcohol and cigarettes. How will the thriving medical / recreational marijuana industry be effected by this change, both positive and negative?"
It’s not exactly clear how descheduling would impact the cannabis industry precisely, but it would likely be a big positive. There are many challenges associated with being a Scheduled drug for this plant, including lack of banking access, onerous taxation (280E) and lack of regulatory oversight by the EPA, among others. All of these would presumably go away.
What’s not clear is how the industry would evolve. Would there be federal oversight and taxation, or would it be left to the states? Will there be restrictions on types of products that can be sold and where they can be sold? It could mean the end of "dispensaries" as we know them. It could also wipe out small producers who are unable to cope with the regulatory oversight and its cost.
One thing for sure is that descheduling would open the doors to pharmaceutical research, a major positive. It would also likely mean that possession of non-commercial amounts would no longer be punishable, a huge victory for social justice.