At a cellular level, the opposite of ageing isn't longevity, it's cancer.
I, for one, would be happy to trade longer life for a higher risk of cancer. As a reasonably healthy person today, I'm pretty sure that by the time cancer as a result of longer life became an issue (say, around the year 2040-50 in my case), the cancer itself would be easily treatable by other means.
What makes you so sure it will be 'easily treatable' by then ?
Am I reading you wrong in understanding that you think that cancer will not be treatable in the mid 21st century, to at least a far greater extent than it is today? That seems the much harder case to make...
Newsflash, we've lost that war.
http://en.wikipedia.org/wiki/War_on_Cancer
We are hoping but we do not know for sure that new directions and avenues researched for treatments will give us new tools to help in finding a real cure.
But for the moment, and probably for the foreseeable future there is no indication that we will actually find this cure. At the moment - as crude as it may sound - if you could choose between being HIV positive or having a malignant tumor I'd pick HIV, we have a much better handle on arresting HIV than we have on stopping a cancer once started, other than by surgery, radiation and chemo therapy and a lot of good will.
Once a cancer has metastasized you are living on borrowed time.
Just like cancer isn't treatable by a much greater extent than it was 40 years ago I would not bet on seeing that change in the next 40 years.
That does not mean we should stop researching or that we will 'just have to live with it' but I think it is a bit early to start putting dates on when a solution will be found.
One of the bigger problems is that the differences between healthy cells and cancerous cells are minute, in some way you could say that the cancerous cells are the stronger ones. This makes going after cancer cells and not killing the cells around them (or even the whole organism) very difficult.
Trading a probability of longer life for a higher probability of cancer is just stupid. Most cancer treatments have the potential to (dramatically) shorten your life -- i.e. the combined probabilities probably don't work out in your favor.
Make the time you have count.