A prerequisite for any vault is unambiguously locking the coins to a certain destination during the trigger stage. CTV is the most basic way of doing this; in fact, anything that is capable of accomplishing this is either CTV or a generalization. So an argument for the value of vaults is in essence an argument for CTV (or something more general that encompasses it as one mode).
One (or three) custodians saying “I’m not going to use taproot, it doesn’t make sense for me” isn’t an argument against taproot. Especially if you have a number of other custodians (who are more current and not working on competing products) saying, “yes, I’d really like this, it would make a difference for my business.”