The only distinction with Medicare and SS tax receipts is accounting. It all goes in a pile, gets spent and a record is kept of what is owed for SS and Medicare. As the Republicans say, an "IOU" - which the Democrats dismiss. Because it's neither officially, but it IS, essentially, because we manage to PAY them every year while running a deficit overall.
It's kind of like having your paycheck direct deposited into savings and checking - with overdrafts automatically drawn from savings. Every month you pay the bills, but it's always more than the amount in checking, so the overdrafts kick in. You tell yourself you're not taking from savings, because you don't move the money, but it happens anyway.
What the Republicans have observed for DECADES is - see, FICA works like this - 7.65% of your income is taken out - and your employer ALSO pays 7.65% - for a total of 15.3%. TRY and imagine HOW MUCH retirement savings you would have, if SS worked like you THOUGHT it worked - if that 15.3% didn't go into a big slush pile but into an investment plan FOR YOU.
I'll tell you what would happen - you'd be freaking rich by retirement. Uncle Sam takes essentially 15.3% of your income, spends most of it, and promises to give you a fraction of it 50 years later. And NOTHING if you die first. Sucks, doesn't it? If it was YOUR money, you could at least leave it to your children.
What the GOP has proposed - is to gradually or partially invest those dollars. The Dems have pointed to the worst collapses in the market is an indicator that the market is not a safe place. Well neither is Uncle Sam. At least the market, over long periods of time, makes money. Actually, the stock market has been the most profitable place for your money over any 30 year stretch in history. It loses big some years, gains others, but ALWAYS outstrips the government in payment.
Yes, the OASI Trust Fund is just a big IOU in the sense that an IOU is what money is. It's what savings are. If you have saved money, you have an IOU (or IOUs). Currency itself is a generic IOU from society at large. (Of course, stored as currency would - in this context - be a completely impractical way for the savings to be held.) And the other ways of holding money are just as IOUs from particular entities. If you put money in a bank account or CD, you've loaned it to someone else. To the extent you have savings, you have IOUs. The same is true for the Social Security program. The question is, who or what is the money going to be loaned to. For a number of reasons, the most practical answer is the federal government.
Money can also be invested, of course, instead of saved. But that's impractical in this context for a number of reasons - not least of which, in my opinion, is that I don't want the government owning private entities and effectively playing an even greater role in picking private winners and losers. I don't want more communism, I want less.
The problem with the Social Security program (aside from it existing to begin with) not having enough savings to carry it far into the future doesn't stem from it having mismanaged the money it saved. The problem stems from it not having saved much money to begin with. For the most part that hasn't been what it was meant to do or what it did. For the most part what it did, and meant to do, was spend the money it brought in as it brought it in. It took money from certain people and gave it to others. To the extent it did save a little money (i.e. collect a little more than it needed), it has actually done fairly well when it comes to return on that money. The great majority of the money currently in the Trust Fund comes from interest rather than operational savings.
Yes, people could (or should be able to) do quite well if they put aside 15% of their income every year to invest and/or save. They should be able to, fairly easily, build a nice nest egg over time. But that scenario isn't comparable to what happens with Social Security - to the point of Social Security. For it to be, they'd have to be taking 15% of their income every year and giving most of it to others who, ostensibly, need it more at the time. Then they could invest or save the 1% that's left. They wouldn't be able to build as much of a nest egg in that scenario.
The major problem with Social Security is the program itself - what it tries to do is the problem. The reality that whatever (relatively) little bit of excess operational funds it's had have been put into treasury securities isn't the real problem. And there's just no good way of transitioning out of this boondoggle, into a private system for the masses to provide for their own retirement, without a lot of pain to a lot of people who vote. Ergo, politically, it's going to be very difficult. What do we do with the older people - approaching retirement or just recently retired - who thought they were going to largely be taken care of through Social Security and haven't already provided for their own retirement security? Who is going to provide the funds needed for them? Again, Social Security is and always was a present day wealth redistribution system. It was never a retirement savings program. You aren't getting your own money, which the government put aside for you, back.
For my own part, I'd be okay with blowing the whole thing up starting today. But I'm lucky, there's a whole lot of people for whom that would be quite problematic.