I ride motorcycles. I have friends and family involved in scuba diving, paragliding, football, rock climbing, hell - even horse riding.
_All_ of those activities have "well known risks of serious injury or death" associated with them, and each of them I've heard recent stories of death in the extended circles of my family and friends.
I'm not sure whether the "designer drug" risk is anything like as high as some of those legal and "acceptable for your children" risks.
My approach is less "asking are the risks of X greater than the risks of making X illegal?", but closer to "Quantify the risks of X, work out which can be mitigated or reduced without needing to ban X, make sure participants are informed of the risks, and if needed ensure training/insurance/licencing is available if required".
Thirty thousand people a year are killed in motor vehicle accidents in the USA. We don't react by saying "Make cars illegal" - we manage, mitigate, reduce the risks (quite successfully, that 30k is down from 40k 10 years ago, and over 50k back in '79 and '80) - then we declare 30 thousand dead people a year an "acceptable cost" for the benefits of private car ownership.
Why aren't we able to be that rational about "designer drugs"?