70 is just about the same as 65 was back in 1935. As written, the act provided only for widows, orphans, cripples, and the truly destitute. Somewhere along the way, the middle class started thinking of it as a retirement fund. Once the Middle Class stopped reproducing at a replacement level, the immigrants started to come into the country to fill more and more jobs.
No it's not. Just because medical science is keeping people alive longer doesn't mean your body isn't breaking down, especially if you do physical work. I know first hand. Baby boomers getting older is a big part of why Social Security disability claims have skyrocketed. I know there's a lot of younger people claiming mental disability too, but most of it's older people with physical problems.