First that old chestnut, that because we are part of nature that anything we do must be natural and therefor OK just does not cut it. Sure Nat Selec produced us - but having elucidated the mechanisms whereby nature opperates we are thus in a position to subvert them and do so on a regular basis - we fly in the face of natural selection by preserving artificially that which would if left to the natural course, die. What we are doing is playing God with the mechanisms and this is so far from being 'natural' it hurts.
The 'cancer' thing is certainly a white elephant because the big C is principally a disease of old age as Vraith points out and those areas of most concern in this argument must be the ones where individuals are promoted into the breeding population who would otherwise not be.
I am very suspicious of the idea that freeing people from the 'drudgery' of work to 'pursue the things they love' is a thing to be desired. Looking at my own situation, I work 40 hours a week as a clerk in a 7-11 store. I grumble at the tedium of the shelf-stacking, the rudeness and ignorance of the customers and at having to get up at 6am and go to bed at 11.30 etc. But were it not for this job most likely I would retreat into a place where virtually all of my human interaction would be via a screen. Five days a week I am forced to get up, to go and laugh, and cry, and complain and sympathise with my fellow man. I suport my work mates when they need it as they suport me. I advise those who are in need of advice, I provide a service that makes me valuable to myself as well as those who utilise my services. I provide for my family and I really, really enjoy my 2 days of a week by virtue of it's contrast with my working days. (Plus I am a keen wine lover and 2 days a week is quite enough both in terms of bank and liver to pursue this hobby!).
Now the 'good jobs' question must be adressed. Those so called 'good jobs' are in reality only good to those looking in from the outside. An example of this would be a British GP (ie doctor) in his surgury. For a year or two after qualifying the job would seem good, until the drugery of sitting in that surgury day after day listening to an endless sucessesion of hypochondriacs, malingerers and downright scroungers began to take it's toll. 99% of what he did would be tedious repetition and most of it of no value to the neurotics and benefit scroungers arrayed before him. Only in very, very rare cases woud he really be of useful help BUT those few cases would make it all worth while. But by God it would still be hard money. Society would deem his job to be of high value, but would the world be so much worse off if he stopped doing it than say a milkman who makes sure 1000 families wake up to fresh milk on their doorstep every day for 30 years. It is a rare thing indeed for a job to be regarded as high grade both by the society and the person who does it in addition for it being loved by that person as a dailt pursuit. Most doctors, scientists, lawyers etc by choice would be doing something else with their days. But the point is that though people may not like it, the having to get up and do a job of work - no matter what it is - is both good for them and the society we live in. To take that away without doing great damage to both people and society would indeed require a 'different type of person'.