According to Robert Ball, someplace in _Straight Talk_, the intent of the 75 year forecasts was political. 75 years was chosen to reassure people that within their lifetime they would get a benefit for their taxes, since 75 years was long enough to cover a concerned individual's lifetime. I didn't understand Ball completely, but apparently at the time showing a 75 year actuarial balance was not that difficult.
I'm not sure what the difference is, but showing such a long term positive balance today is problematic.
I think it should be abandoned too, since it now fails its original political intent, quite apart from being absurd on its face.
-- Homines id quod volunt credunt.