If I want to be blissed out until my body falls apart, I can do that now by turning my retirement account into a stockpile of fentanyl. I don't think its presumed that intelligent civilizations all just do this, for the same reason I don't believe modern human civilization will collapse on itself simply because we've discovered opium.

The key difference is that we still live in a society where, at least most people, have to work to live. If you spend your retirement on fentanyl the fentanyl isn't going to be the thing to make your body fall apart, assuming you get pure shit and are able to dose properly and not od your body can handle that for decades. What's going to tear your body apart is the poverty and deprivation of living on the streets after you lose your job. If you're in a fully automated post scarcity society and you're able to hook yourself up to one of these machines and live a long life I could see a majority of people doing that. Sure some people would object to it being meaningless, but in a post scarcity reality where God is dead, a robot can do anything better than you, and there's no conflict or competition for resources there isn't much meaning to be had anyway.