However this also ties in with eugenics: if we euthanize people with severe disabilities, and only allow the "strong" to breed, eventually we can move on and eliminate most disabilities right?
It's not that this idea sounds cruel and inhuman, it's that it *is* cruel and inhuman. There is no "seeming" no "sounding" nor any "maybe" about it. Eugenics has been rejected for several reasons. First and foremost, who gets to pick the "defective" traits? I know people who think that garden variety stupidity is enough. Then, I am sure, there are those who think that specific races or "the Lebanese" are "inferior."
Again, though, power tends to corrupts, so I would not expect those in a position to grant life or death upon those around them to wield that power fairly forever. I certainly would not leave the decision of what is genetically useful to the majority, and leaving it to an individual would be even worse.
Suppose we take something like severely impaired intelligence, is that sufficiently inferior to weed it out of the species? If we ever find ourselves in a food crisis, a lack of intelligence may be a survival advantage. Brains are expensive things, requiring lots of protein and consuming a huge number of calories. It's inevitable that there will be a global crisis on the Earth some day.
Second, and more importantly, we already have natural selection functioning, so who the Hell needs euthanasia? What's the rush? Why do we need to intervene at all? If the traits a specific individual has are truly maladaptive, natural selection will take care of them, and if they are *not* truly maladaptive, then there goes the whole premise behind euthanasia. (Or is it? In general euthanasia proponents have not been concerned with the enhanced survival of the species so much as with eliminating traits they did not like. That said I see no reason to be against dumb people under starvation conditions to the extent they are out-competing the smart people.)
Thirdly, you also have to bear in mind that euthanasia, even when targeted at a clearly maladaptive trait with no redeeming offsetting values is like using a shotgun to kill a mosquito. You get the bad trait all right, but you also kill off countless other genes, most of whom one presumes were perfectly fine. Gene therapy is eminently superior to euthanasia, so you should focus your efforts there, if anywhere.
Fourthly, who's to say that a happy life isn't worth living? Perhaps a disabled person will never do the things a non-disabled person will do, but perhaps he or she would find happiness anyway. If you feel pity/repugnance when you look on them, that's more of a reason to kill yourself than to kill them. If you think they are less likely to be happy, so are people born into poverty, statistically speaking, and we don't put them to death. We have long since flipped the evolutionary paradigm from maximizing "reproductive success" to maximizing utility (personal happiness) in western culture.
So, it seems to me, that hypothetically, so long as the expected net present value of the a person's current and future utility (happiness) is > 0, there is still no need to kill them under that rubric. Their life overall may not be as happy as you will be, but yours will not, I guarantee, be as happy as Michael Jordan's. The
relative happiness does not matter so long as the next amount, properly discounted, is still positive.
There is yet another non-survivability rubric under which we value only their net expected contribution to society. If we expect them to take more than they given, we eliminate them. If they are expected to give more than they take, we keep them. I don't believe that you propose that as the measure, though, so I ignore it save to say that I find it unappealing. To boil a life down to "have they done good by the State" is, to me, appalling since the question should be reversed...what has the State done for them.