You wrote:
They're money market funds so are covered by SIPC insurance.
If I understand your post correctly, this is not like FDIC insurance. Money market funds can
always lose money and are not SPIC insured like FDIC insurance. (Please correct me if wrong.) What does it mean to "lose money" in money market funds? Most money market funds are buying very short-dated debt (about 90 days or less) -- like commercial paper [CP] (very popular before 2008) or US Treasury bills. For CP, these are not principal guaranteed, like US Treasuries (please ignore the Tora-Bora cave dwelling DeFi crowd that worries about the US Federal gov't going bankrupt -- I expect there will be a few replies to this post!). As a result, it is possible for the CP issuer to go bankrupt and be unable to repay the principle borrowed.
I think you are confusing SPIC insurance that covers you if the brokerage firm files for bankruptcy. No matter the losses from a brokerage firm, you will be covered by SPIC insurance as:
The limit of SIPC protection is $500,000, which includes a $250,000 limit for cash.
To be clear, money market funds are
not cash. They are consider securities under US securities laws. You will be covered up to 500K USD. In my personal opinion, they are about 95-98% as safe as US Treasuries -- usually a very good investment.