My understanding, or at least why I don't worry about the $500k general limit.
When I hold, for example, the mutual fund VTSAX inside my brokerage account at Vanguard, I don't really have exposure to anything. Those shares are held by a separate custodian that is audited (leap of faith there of course that the audit system works). If the Vanguard brokerage platform was hit with something that caused it to bankrupt, I still have my shares of VTSAX. They would either be sent to me or, more likely, be transferred to some replacement brokerage.
If I had my money sent blindly to Madoff Inc., which was "self-custodianed", well, that's where I would expect to have exposure that SIPC might be needed.
I have a bunch of shares of VTSAX, and I have every confidence that about 5.6% of my current balance could be traced to actual shares of Apple in some ledger on some cloud.