If 100 year olds were truly healthy, then I strongly suspect we'd see a shift in societal structure and expectations for retirement. Societies tolerances for a permanent upper class of retirees and a permanent lower class of workers would likely not last - worker's would likely be unwilling to do the same job for a century.

If people started to live longer, they would (hopefully) be in better health longer, and retire at a later age.

A better question might be can we afford NOT to have life extension? Advanced economies all over the world are filling up with sick, elderly people who form a massive drain on the economy, and it's going to get much worse. Developing medical technologies that can keep them healthier and productive for longer is the one thing that can save us here.