1) To get to that number the Trustees assume fertility rates will rise to over 2.2, a rate not seen since 1970 which completely goes against the trend of the last 50 years.
2) They continue to assume 1 million migrants a year to the US, despite active legislation seeking to limit migration to half a million a year.
https://www.ssa.gov/oact/TR/2016/2016_Long-Range_Demographic...
All of this is dependent upon a Labor Force Participation Rate not encumbered by automation and a general lack of skills by the populace. Which may not be a problem but the BLS has been underestimating the decline in LFPR in their projections for the last 20 years. We're decades a head of where we should be based on Demographics alone, according to all of their predictions.
https://www.stlouisfed.org/publications/regional-economist/o...
Eh, if it makes it past the Boomers (which it does in the low-cost estimate in recent trustees reports, but not in the intermediate- or high-cost estimates; so risky-but-plausible with no intervention, certainly achievable with minor tweaks) it's fine, demographically, for the foreseeable future; Gen X is a demographic trough, and the Millennial generation is a gentler boom than the Boomers.