Prohibition is what is causing this problem in the first place.
If you legalize marijuana, would be no profit to be made in coming up with new synthetics and without a profit motive, only researchers would do it.
For example, since the end of alcohol prohibition, how many cases are there of people being blinded by methyl alcohol from bathtub gin?
It came from an argument (in my head ;) based on the proposition that you would ideally internalize adverse effects in the price of a good. So alcohol-users would pay for alcohol related mischief, LSD for LSD, etc. You would need a product specific VAT for the larger categories. Can't really implement that for N designer drugs, so I thought 'ban the rest for easy of acceptance'. Your effect is more beautiful though.