Complex sentence: Some argue that since Social Security taxes are eventually returned to taxpayers, with interest, in the form of Social Security benefits, the regressiveness of the tax is effectively negated--i.e., the taxpayer gets back what he or she put into the Social Security system.
Split into simpler sentences: Some argue that since Social Security taxes are eventually returned to taxpayers, with interest, in the form of Social Security benefits, the regressiveness of the tax is effectively negated. That is, the taxpayer gets back what he or she put into the Social Security system.