The reason there's a problem is that people are already refusing to die on schedule. Pensions won't 'go away' by magic. People don't want to work till they drop, especially if it's for 200 years.
No, they won't go away by magic, they will go away by economics. Actually, they're already going away, as the industrial base shifts to newly developing nations and old companies give way to new corporations with alternative labor arrangements.
By the time real anti-aging comes along, most of the people who were entitled to pensions will already be dead. Those who aren't will probably find their former employers going to court with the claim that if a person is effectively immortal, it violates the underlying legal assumption of the pension. Especially if that person is no longer "elderly", but now fit as a fiddle and able to work. Should be some interesting court cases, actually. Bottom line, the laws around retirement age and retirement savings plans will have to change.
as for the work thing, well that's tough. Unless a Trekkian economy develops (and who the hell knows how that would work), adults will still have to earn a living for a good portion of their lives. Immortality doesn't mean a life of endless leisure, although it could allow you to pursue a number of different careers in your lifetime, maybe taking breaks in between and living on the savings.
Some would consider that rewarding. If they don't, they don't have to take the treatment. That's probably how it wil ultimately work out; you will have natural lifespan people, and really long lived people. Society will look very different.
(And then the slow aging people will all decide to get pointy ears, and move to a desert planet.)
I don't think anyone has been arguing against extending lifespans, have they?
Well if you aren't arguing against it, then I don't understand where you're coming from. Essentially this is what I'm hearing:
"Life extension will cause overpopulation, people don't want to work forever, it will bankrupt retirement plans, etc. - but I'm not arguing against it."