What you've stated does make sense. The only difference I see is that removing the need for a drug dealer or a pusher and letting a legitimate business step in would cause a decline in secondary crimes related to the legalized substance, i.e. rival dealers, turf wars, muggings during drug deals etc.I've have heard this a lot.
Using a similar thought process one might argue that we could reduce automobile and motorcycle tickets and fines (and insurance increases arising therefrom) by taking down all speed limit signs and stopping the enforcement of speeding limits. If we were to do so, the symptoms (speeding tickets and the cost of enforcement) of the problem would be reduced, but the problem itself (speeding related vehicle accidents, injuries and death) would not also be reduced (and may be worsened). Reducing the crime of speeding would do nothing to reduce the associated costs to society.
That is sort of the way that I view decimalization of drugs. There would be less arrests, but there would be no less (and possibly higher) costs to society. Make sense?
Now there are certainly still instances where this causes crime simply by having a new business that can be robbed but that is in itself a different issue.