FICA taxes are not an income tax. They are a tax on gross receipts. Income is your receipts less expenses, deductions, exemptions, and credits. Your employer withholds FICA taxes on all payments to you regardless of your expenses, deductions, exemptions, or credits. It is therefore a tax on your gross receipts from employment.
Furthermore, FICA taxes are not deductible on your federal income tax return. Therefore, the people who pay only FICA taxes can cry me a river. When I earn $100, not only do I only get to keep $85 of it due to FICA taxes, but the full $100 counts toward my earned income on my federal return. Therefore, FICA taxes are more onerous for income taxpayers than for nonpayers because of the nondeductibility.
Finally, the purpose of FICA taxes is to fund 2 very specific federal programs, and the argument here is not whether payers of FICA taxes are entitled to the benefits from the programs they have paid into. The argument is whether people who only pay FICA are entitled to the benefits from programs funded through the federal income tax. In my opinion, the answer would clearly be no if the government were actually keeping FICA taxes in a lock box type system. Since that's not the case, and FICA taxes are actually used to fund the general operations (and soon to be vice-versa), then maybe there's an argument to be made that FICA payers have earned other benefits. In my opinion, that's still pretty weak.
Capital gains are an income tax. First of all, capital gains on assets held for less than 1 year are taxed at the individual's ordinary income tax rate. Second of all, all capital gains earned by corporations are taxed at that corporation's marginal income tax rate regardless of whether they are short term or long term. Third, losses from capital assets can offset ordinary income up to certain thresholds, with carryforwards (just like net operating losses from ordinary income can be carried forward). Fourth, when the income tax was originally established in 1913, all capital gains were taxed as ordinary income. There are plenty of other areas of the code in which congress has given preferential treatment to certain types of assets - I don't understand why this preferential treatment should exclude something from being called an income tax.