I'm speaking more about devastating to infrastructure. Social systems (social security, employment rates), healthcare availability, etc.
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.