Compare a custodian's utxo set to the custodian's liabilities.
I understand what you're saying, bitcoin is obviously much easier to audit, I just can't see any reason why sufficient numbers of people will ever care.
Discussion
I agree.
I also fail to understand why I should care? If they want to invest their fiat into bitcoin, then never actually withdraw it, so?
Stupid? Yes.
Pointless? Yes.
My issue? No.