Something like: "I understand that this presents a challenge, but we have to test and support this code while continuing to meet our other commitments, and this represents our best estimate for the time required to accomplish this work."
This is a thought-terminating cliche (https://en.wikipedia.org/wiki/Thought-terminating_clich%C3%A...), i.e. saying nothing politely. It contains no information; its purpose is to close that topic and pivot to a new one - discussing a different team doing the work, changing the ask, hiring contractors, etc.
> Ideally one that results in the customer understanding that they're crossing a boundary of sorts here.
A great low-conflict way to convey this is by being more formal/icy in your phrasing than usual.
This is a thought-terminating cliche (https://en.wikipedia.org/wiki/Thought-terminating_clich%C3%A...), i.e. saying nothing politely. It contains no information; its purpose is to close that topic and pivot to a new one - discussing a different team doing the work, changing the ask, hiring contractors, etc.
> Ideally one that results in the customer understanding that they're crossing a boundary of sorts here.
A great low-conflict way to convey this is by being more formal/icy in your phrasing than usual.