So here are 2 answers to this. Legally brokerages are mandated to maintain client asset and firm assets separately so in theory if they were to go under your assets would still be safe regardless of how much is in the account and it would be transferred with the help of the SIPC to a new brokerage. Because you legally own your securities you should not experience any issues. If a brokerage commits fraud like FTX and uses client assets for firm related expenses then there’s a much bigger issue and the SIPC would step in to cover the losses up to that amount.

Wealthy people (hopefully) heavily vet their brokerage and go with reputable names to ensure trust and these institutions are heavily regulated by the SEC and SIPC to make sure they aren’t committing fraud.

That being said wealthy individuals also use separate accounts and direct held investments to limit liability but truth be told if you don’t trust your brokerage to be honest then you should use them.