When LG and I did argue, my concern was never about a slippery slope to "life ends at 80" or something like it. I just don't see society ever turning to think this way. Besides, as health and care gets better, the age at which anyone starts being "just a burden" will become higher.
Rather, I am concerned about the fringe cases, and how legalising euthanasia and assisted suicide might change the way these cases are viewed. At present the system has a blanket illegality, but CPS guidelines lead to very few prosecutions despite this. The pressure however is to ensure that each case is thoroughly investigated to ensure that there was no wrongdoing. I feel that with euthanasia being legal there might be less investigation.
Currently there seem to be no more than 20-odd cases per year where assisted suicide/ euthanasia incidents go as far as the CPS. I don't think that we would see this rate rise all that much even if/ when the Right to Die becomes legal. Meanwhile, even if this is just a fraction of the number of cases that actually exist then we clearly already tumbled down that slippery slope a while back -- so that the only thing new would be recognising that we have. Either way, I don't feel that the slippery slope argument is worth pursuing.
http://www.cps.gov.uk/publications/prosecution/assisted_suicide.html