Mature "legal" markets like Holland surely prove this. Coffeeshop weed is great and has been for decades. People still grow their own, but really don't need to.
Coffeeshop weed in the Netherlands costs about the same as black market weed in the neighbouring countries (that would be around USD 12 per gram at least; that's USD 330/oz). If, say, Germany, would ever get around to liberalizing the market, about half of the current BM price would probably be taxes.
So, yes, people don't need to grow their own in the Netherlands, but it still is very profitable; you will probably have to export the stuff, however.
I think in the Netherlands the weed is not officially taxed, because it's not officially legal (only "tolerated"). The coffeeshops probably pay a hefty tax some other way which keeps prices high.
And I don't see this changing much even in the medium term. Most EU countries haven't even gotten around to liberalizing, let alone legalizing weed. If they do, they will want to collect. The black market will remain, and it will still be huge. I do not see how the authorities could crack down on a black market for weed when growing for private consumption is legalized; you can no longer fight the providers efficiently.