Setting any rate of tax on marijuana will still push it above the prices that it was previously sold at, leaving a large gap in the market for illegal dope.
That could only be true if there were no extra costs involved in selling black market goods but there clearly is. If the tax is outrageous (and I mean outrageous) maybe a few people would keep going to their dealer, but certainly not the majority. Maryjane is ridiculously easy and cheap to grow and turn into something smokeable. Cheaper than cigs. Growers and sellers can make buckets of money, taxes can be relatively....ahem...high and it would still be cheaper than going to the local thug or old hippy is now. And you could do so without fear of police or getting robbed/screwed by your dealer.
Clearly you don't like weed, and that's fine, there are valid reasons to be against it if you're into restricting the freedoms of others, but don't make sh**t up.