As an Australian who had friends who bought product on silk road my understanding was:
1) It's safer to buy something online and have it mailed to your house than go pick it up from some shady dude.
2) On the street you would often get duds or spiked product, online reputations were built up over time and important to be maintained (think uber/ebay stars).
Overall silk road probably increased the amount of drug activity but made each incident safer. Not sure what the overall impact would be.
An 18 year old lad from my village, who had just started a job programming, bought a drug from an online “pharmacy” and it turned out to be spiked with a synthetic opioid (N-pyrrolidino-etonitazene) and he died in his sleep at home, alone.
On your point about spiked products - it’s clearly a problem for online illegal drugs as well as those bought on the street.
The problem is, you don’t get to leave a bad review if you’re dead.
1/5 stars. Quick and discreet delivery. Minus 4 stars because it killed me.