Exactly! And that's the real problem. The real problem is not that SSA invests in Treasuries. The real problem is congressional overspending and over-borrowing. The real problem has nothing to do with SSA or the SSA trust fund.
SSA could invest in China, and Congress would just borrow from China instead of SSA. It would clear up the silly claims of "No SSA trust fund exists". I'm sure China would be happy to stand in the middle for a small percentage. But then you'd be forced to focus on the one really big problem which is congressional overspending and over-borrowing.
Now there is a secondary problem in that the SSA trust fund is not nearly big enough to fund all the future claims against it. But it's nothing but fear mongering to say the trust fund doesn't exist. It does and there is almost 0 chance that Congress will ever refuse to honor the SSA trust fund claims. At best their might be a temporary delay when we hit a debt ceiling and congress can't agree.