According to a 2010 Congressional Research Service report, if all earnings were subject to the payroll tax, but the base was retained for benefit calculations, the Social Security Trust Funds would remain solvent for the next 75 years.
Since 1982, the Social Security taxable earnings base has risen at the same rate as average wages in the economy. However, because of increasing earnings inequality, the percentage of covered earnings that are taxable has decreased from 90% in 1982 to a projected 83% for 2014 and later. Because the cap was indexed to the average growth in wages, the share of the population below the cap has remained relatively stable at roughly 94%.
By removing the cap, as much as $100B/year can be added to the fund, ensuring its solvency through the year 2087 and beyond.