It deserves careful attention. By 2029 the accumulated surplus will be exhausted. Other assumptions, likewise, can be wrong because projecting for so long a period for so large a system is very sensitive to even minor adjustments in the assumptions. This core business of Social Security is an intergenerational and interpersonal compact, reflecting both individual and communal values. Every industrial country has something similar except Japan, which relies on private savings and families.
Hybrids can be the best of both parents or the worst, sometimes both.
Disability is relatively new and much more complex than old age and survivors programs, especially in the establishment of eligibility. If you return to the original discussions when this program was being designed you will be reminded of something current critics gloss over. This system was designed by the middle class to defend itself from the poor. Knowing that the middle and upper income groups were going to get stuck with the bill supporting the elderly poor in their declining years, because this group was not likely to save for old age, a purpose of the program was to force the working poor to contribute something while they were working. The lowest income recipients, or those not covered, are shunted off to SSI, which replaced Aid to the Aged, some disability, and General Assistance in the 1970s.