"If I'm holding 100% securities, a) SIPC offers me no protection from losses"
Sure it does. The point of SIPC is that if your broker goes under, and you had, say, 100 shares of MSFT and $100 in cash, you get those things even if your broker somehow comes up short. Now, what 100 shares of MSFT is worth in dollars is unrelated to SIPC and depends on the stock market.
The purpose of SIPC is to protect against a breakdown in financial abstractions at one particular level.
Sure it does. The point of SIPC is that if your broker goes under, and you had, say, 100 shares of MSFT and $100 in cash, you get those things even if your broker somehow comes up short. Now, what 100 shares of MSFT is worth in dollars is unrelated to SIPC and depends on the stock market.
The purpose of SIPC is to protect against a breakdown in financial abstractions at one particular level.