Voluntary euthanasia is ultimately challenging because of similar legal issues as with the death penalty - it cannot be undone, and there are forces in society that can lead individuals to use it for other reasons than just being over and done with suffering through old age.
> 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.
The death penalty does not exist to reduce the suffering of the convicted, but to get rid of them. The true issue with the death penalty is that it can't be graduated (except by adding "cruel and unusual punishment") and it can't be undone. Prison sentences can be legally challenged and the innocent can be freed early.
There is a real slippery slope here: what length of prison sentence is considered to be worse than the death penalty? An additional thing to consider is that many countries without the death sentence actually don't impose true life sentences, but very longish ones (upwards from 20 years). Confinement for life is for those irredeemly judged to be a threat to society after their sentence. Compared to that, many death row inmates actually spend decades fighting their sentence. They could end it at any time if they wanted.
> The death penalty does not exist to reduce the suffering of the convicted
There's an easy way out of your moral dilemma that you go into after this sentence, much like what I suggest for those on the outside: let the convicts choose whether they want to suffer for the rest of their days in prison, or be humanely and painlessly killed. I know which way I'd go, under the circumstances. And yes, I do insist that the killing must be humane, dignified, and painless. We have the technology to ensure all three of those things.
And longevity research is not even a real need - we already live too long as it is, from the evolutionary and economic standpoint. I'd much rather someone came up with a way to cheaply and painlessly end one's life once quality of life begins to deteriorate due to chronic disease and such. Some kind of company (and legislative framework) where you pay, say $1K and they put you into a nitrogen chamber and then cremate and flush the ashes down the toilet afterwards. Or perhaps use them as fertilizer. I'd use the service myself at some point in distant future.