It was intended to be funded in real time by FICA taxes generated through both employees and employers (many employees do not realize that their employers pay half their FICA).
It was also intended (all the way back to the beginning) to be a supplementary income for the elderly - and back in the 1930s, you were elderly if you made it into your 60s - with the intention that you would have other incomes or family taking you in and supporting you. It was basically designed so that people would not have to live in abject poverty in their old age.
When the government talks about the reserve fund being depleted sometime around 2035, give or take a few years, we will rely completely on those incoming FICA dollars to pay out Social Security benefits. That's why the benefits go to about 83% instead of to zero. But over time, as there are less people working to pay for those collecting benefits, that percentage will continue to drop until one or more of the following happens, which it will, because it is politically unviable for those in power to allow the elderly having their benefits reduced. Of course, being politicians, they will continue to kick the can down the street as long as possible, hoping that it becomes somebody else's problem. In the meantime, they will thrive on scaring us and trying to accuse Republicans of pushing granny off a cliff.
The last option is the most incendiary and least likely as people pay into it all their lives and expect something back, even if it is a small part of their total income. But it is almost a given that the minimum age you can collect gets raised from 62 to a higher age.
Another likely scenario is that the income cap at which FICA is taxed on is removed entirely. Currently, if you make more than $168,000 this year, any future income will be minus the FICA tax. That cap gets raised each year but look for it to be removed entirely or put on some kind of sliding scale.
For all those who like to say "Just give me back what I put into it and remove me from the system", it just isn't going to happen. For one, the money you put into it years ago is no longer there! For another, we are still going to have to pay those who are collecting, whether we benefit from it or not. It sucks but that's the way it is.
I detest politicians, by the way.
They also could raise the FICA tax from 15.3% to something higher for both individuals and employer.
See:
https://www.fool.com/retirement/social-security/benefits-formula/#:~:text=Social%20Security%20benefits%20formula%202023&text=Multiply%20the%20first%20%241%2C174%20of,to%20the%20next%20lowest%20%240.10.