Do Thai authorities think that making it illegal people will stop smoking? If so, I'd like a couple puffs of whatever they are smoking. Where do they think all the ganja from producers will go? The government isn't going to reimburse investors who built grow labs, developed a distribution networks and invested in shops.
What will happen is those who want to get high will buy from the black market. So much for the VAT the government gets from production and sale of ganja. Many folks working in the dispensary side of things will be out of work but I guess the government thinks good jobs are everywhere so they will quickly find something else.
Lastly, remember what it was like in Bangkok priot to Prayut and his gang taking over. Many reports of shakedown by cops with fake pee tests and "finding" a yaba pill in a tourists pocket. The BIB will soon be adding to their take with bribes and kickbacks from tourists who still think it is legal in Thailand to smoke pot.
Another fine example of Thai politics.