That's the point. As long as I'm lucid I should have full bodily autonomy, including the decision to shuffle off this mortal coil. In fact I already have control over this decision.
> and there are forces in society
So? You're going to tell me I can't go anytime I want to? That's not the case even now. It's just that now I'd have to procure the nitrogen myself (which isn't difficult), and my relatives would have to deal with the body. I'm merely suggesting a service that resolves this purely logistical complication, and excludes the possibility of not quite dying but living the rest of one's life as a vegetable.
Think of what we have now: people spend years, sometimes decades suffering from chronic diseases, or just plain not having anything or anyone to live for. And it'll get worse as medicine "improves", and lifespans "improve" with it. Is it humane to withhold the option to end it all from them? I don't think it is. I will grant you that there are likely tens of millions of such people on the planet right now. I will also grant that this is not an uncontroversial thing to suggest. But the alternative we have now doesn't seem any more humane or dignified to me.
If this still doesn't sit right with people, we could age and condition-restrict it, or require a long waiting period for when this is not related to acute incurable disease.
> as with the death penalty
Which is also inhumane, IMO. It's much worse to spend the rest of one's days in confinement instead of 30 seconds until barbiturates kick in. That's what the sadists who are against the death penalty are counting on.