I hope we're really considering the effect of widespread longer life might have instead of barreling toward longevity - people living 10 years longer on average could have a pretty devastating effect on a lot of things. And I'm not saying we shouldn't do it, but we should be really thinking about how we'd mitigate that impact.
That's sarcasm, of course, but IMHO saying that we shouldn't prolong life because of possible consequence X is morally exactly equal to saying that we should kill old people to avoid that consequence X.
For every X, either X really is so bad that it's worth killing people for it; or X shouldn't be considered as a valid argument against longevity.
Could you say a bit more about what problems you foresee? Are you talking about over-population, because I don't think we are at all close to that point. Other than medical costs and systems built around an expected retirement age I don't see any problems.