When I was nearing my ER date in 2008, I became a little concerned about the $500k SIPC limit. However, Fidelity has excess insurance coverage of assets which made me feel better.
Back in the 1990s I had a more even split of investments (excluding my 401(k)) between Fidelity and another company. But in the 2000s, after a few bad returns in the funds of the other company, I stopped investing more money with them. Still, I am glad to have not quite all of my eggs in a single investment company basket. If something strange happens with Fidelity and I can't access my money from them for a short time, I can still go to the other company to tap into funds in a pinch, like an "emergency brokerage company."
Retired in late 2008 at age 45. Cashed in company stock, bought a lot of shares in a big bond fund and am living nicely off its dividends. IRA, SS, and a pension await me at age 60 and later. No kids, no debts.
"I want my money working for me instead of me working for my money!"