There are different types of feeling good, and the purely chemical good isn't particularly rewarding. There are the moments in life where you are happy because you are living the life you want to live, and those are way better than just feeling good from drugs. Sometimes life goals and drugs come together, and that gets really interesting. It's reasonably common for people to do MDMA and have a glimpse of their life without anxiety and trauma, or LSD and looking at their own existence objectively.
For me, the best moments in my life are around building connections with people.
Not trying to single out parent, just felt like it should be mentioned.
It's sort of like the fact that nothing tastes better than cheesecake (or insert your favorite dessert here), but that doesn't mean you can't enjoy life or even other food without it.
As far as drugs go though, there is a lot of personal preference involved too. For me, MDMA in unhealthy quantities is still the best that I've ever felt, and I know some other people prefer the meth high, but I don't think that's exactly what you are arguing against :)
I suppose this varies a lot from individual to individual. I had morphine a few months ago for a shoulder reduction, and I felt horrible enough that I almost wish I'd just done it without painkillers at all.
The evasion and mental gymnastics does have some point at least - it isn't the doctor deciding to take the life of a patient because of a judgement and well any student of the 20th century knows how /that/ can end.
But there are lots of drugs that have the possible side effect of death, and plenty of patients who decide that the risk-benefit of taking a drug is warranted even though it might kill them.
So therefore, everything has to be described in terms of "pain management" and "comfort" where the cessation of breathing is just a... potential side effect. If that, combined with the patient's standing orders not to intubate or perform any sort of resuscitative action in the event that they stop breathing, leads to death... well...
The euphemism that is frequently used is "palliative sedation" (or "terminal sedation", although that seems to be used mostly by people of the thou-must-maximally-suffer school of thought).
I'm sure that's not true of every user, but enough of them that I really think heroin(and strong opioids in general) is a different beast.
edit: I'm being rightfully called out. I haven't done heroin and didn't mean to authoritatively say that there is only one realistic outcome. I only meant that painting the picture of drugs completely ruining your entire life isn't accurate, we all know very successful people that regularly do drugs.