But your team clings tenaciously to a broken design. Why?
Maybe we're seeing the beginning of the "I'm going to do exactly the opposite of whatever the NSA tells me to do" era. The NSA says to use a MAC, so I'm not going to use a MAC!
Hey, but now he knows that you are planning to do the opposite of what he says, presumably he will start giving you good advice, just to trick you.
The reason that people are so cynical about your custom solution is that being completely and utterly cynical about custom solutions, unless the architects can defend the solution rigorously, is the only sane approach in cryptography.
Maybe we're seeing the beginning of the "I'm going to do exactly the opposite of whatever the NSA tells me to do" era. The NSA says to use a MAC, so I'm not going to use a MAC!