At a systemic level, this is dependent on what "normal" purity is for users. First-time buyers on darknet markets probably are more likely to overdose because they're used to less-pure products, although I don't have any statistics to back up that guess. But if people are buying on the darknet consistently, they'll be unlikely to overdose due to unexpected purity (though they might still overdose for other reasons).

I'll admit I haven't done much research on opiates specifically for the simple reason that I have never known any active opiate addicts (though, I did get trained to administer Narcan). However, in my understanding of drugs such as coke, MDMA, or speed/adderall, which are more common in the tech scene, higher purity is unambiguously a net positive. It's been a while since I was actually involved in the overlap of the tech/festival scene but when I was around that more, I made anyone I knew used drugs aware that I had drug test kits and would let you borrow them no questions asked. I can't claim I ever saved a life, but I can say for certain that ~30 people at a festival I went to ended up riding out bad trips in medical tents or being transported to the hospital due to MDMA cut with DOC, and none of the people I let borrow my test kits at that festival did.