The trouble is internal. I'm having a hard time seeing how to structure it so that only certain people have access to the keys while preventing theft. Not to mention the chance of tech failure. Way too much risk.
Discussion
Multi-custodian multisig would be using big, professional custodians to hold one key each.
Something like Coinbase, Unchained, and Block each get a key and 2/3 need to sign. Maybe even 3/5 with two keys being internal. That would be a great trust distribution if you ask me.
We have to remember - we aren't inventing magic, we are trying to fix the legacy financial system. Banks will exist, and lots of people will have trusted relationships.
How do corporations hold money today? In shit paper at fractional reserve banks with a zero percent reserve ratio. Hard to get worse than that.
So for public companies, custodians are essential, at least for now. While I think having Coinbase be the sole custodian is foolish, a custodial solution really is necessary.
It's a matter of trust. I think there are many companies which could make a self custody protocol and implement it successfully.
But where you're dealing with hundreds of billions worth of Bitcoin, you can't leave that up to a single person or even a small group of people.
It needs active physical security, as well as professional cyber security and risk management. Having specialized professionals which handle this makes sense and there will aleasy be a market for it.