Social Security serves as a critical source of income for millions of retirees who would be forced into poverty if it weren't for those benefits. And 39% of seniors in their 70s regard Social Security as a lifeline, according to a recent survey from TD Ameritrade. But despite the importance so many of today's seniors place on those benefits, if you're years away from retirement, you may be better assuming they won't be there.

Don't bank on Social Security

Social Security is grappling with a host of financial issues. In the coming years, its projected obligations will outpace the revenue it takes in, largely because of the massive number of baby boomers who will be exiting the workforce before an adequate number of replacement workers are able to enter it. And while the program does have trust funds it can tap to make up for revenue shortfalls, based on recent estimates, those funds will be depleted by 2035. At that point, Social Security may need to cut benefits if Congress doesn't find a way to get the program the money it needs.

Social Security cards loosely stacked

IMAGE SOURCE: GETTY IMAGES.

But that's not the only reason you shouldn't count on Social Security for your retirement. The bigger reason is that those benefits are by no means enough to live on.

The typical senior on Social Security today collects around $1,500 a month, or $18,000 a year, in benefits. Now, think about what your living expenses amount to on a monthly or yearly basis, and assume you'll need 80% of that total in retirement. Chances are, Social Security won't come close to cutting it.

In fact, the average worker today spends about $5,100 a month, or $61,200 per year, according to the U.S. Bureau of Labor Statistics. At 80% of that total, you'll need $4,080 a month, or close to $49,000 a year, to pay your expenses as a senior and live in a reasonably comfortable fashion. As such, Social Security clearly won't cut it, and if you decide not to count on those benefits, you'll be more motivated to start saving money on your own.

Secure your own future

To be clear, Social Security isn't going away, so when we talk about writing off those benefits, it's not because you won't be collecting them in some form. Rather, it's because many people aren't naturally motivated to save for the future, so if you convince yourself that it's truly necessary, you could see better results.

Of course, to save for retirement, you'll need to make sacrifices in the near term. You may need to spend less on leisure, take fewer vacations, or drive a used car instead of fancy new one to free up money for your IRA or 401(k). But if you don't make that effort, you'll risk falling seriously short in retirement.

The good news is that if you're many years away from that milestone, you can build a decent-sized nest egg without having to part with huge sums of money each month. In fact, if you sock away $420 over the next 40 years, and invest your savings at an average annual 7% return, which is doable with a stock-heavy portfolio, you'll wind up with $1 million. And if you manage that sum wisely, it should be enough to get you through retirement without having to struggle financially.

Social Security benefits may or may not get cut in the future -- but either way, they can't sustain you as you might expect them to. While it's OK to factor those benefits into your retirement plan, you may be better off pretending they won't come through for you. That way, when that doesn't wind up being the case, you'll have bonus cash to play around with in retirement on top of the solid nest egg you managed to build for yourself.