Those "massive assets" are T-bills.
If push comes to shove, who do you think will get shafted - SS or China?
More to the point - how much are you personally willing to pay to maintain the SS benefits for folks 10-20 years older than you? How much do you think that folks who are 10-20 years younger than you are willing to pay to maintain your SS benefits?
I really don't want to see a stock market that has 3x the money coming out as is coming in.
T bills are a claim on future US tax revenue. That's very different.
What happens to the prices of those assets when the baby boomers retire and are pulling out more money than there are young workers putting in? You have the same problems as SS. What if we had all of that SS money in the market? It'd be a disaster.
WRT companies that produce stuff, the critical time is not when boomers retire, it's when they stop spending money. Fortunately, many will stop when they die, which reduces demand and the "load" on both the stock market and SS. Again, the whole world thing is relevant.
And, since boomer spending is income for later generations....
Do you really think that money invested in the stock market is like SS obligations? (Here's another difference - Congress can repeal SS whenever it feels like it.)