I get your point about custodial accounts being safe but the one operating such service is what most people are worried about. While true that personal accounts on such services cannot easily be compromised due to a number of security features and all other implementations, there's really nothing a user can do if the whole service itself gets hacked. No safety nets, no guarantees, no assurance--just another creditor shouting scam and just waiting for whatever money the service can give back as means of 'honoring' their word to avoid jail time.
But it can be done in an auditable way, custodians can keep the coins in cold storage and sign with their addresses to prove that they have coins, then auditors just need to look at the client's investments and verify that it it all balances. And with schemes like multisig, you can have regulators hold one of the keys, so that the custodians won't have a temptation to steal the coins.
Let's say that that's the case. Complete audits, the service has weight and enough credibility and money to operate. At the end of the day, data leaks and inside jobs can still happen and you are still doomed to fall under a sad, sorry state. Even exchanges with the 'best' security implementations and features such as multisig wallets and cold storage have fallen victim to hacks and inside jobs. The point is, if the price is right, operators will risk it--even the regulators can be an accomplice.
Personally, I prefer depositing and taking my coins without any hassle or whatsoever, which is why I will always choose an exchange. The liberty it offers to
my coins is simply preferable rather than having someone hold your coins for you, and if you want to take it out, you have to do it on their terms. Whatever floats your boat, I guess.