“We'll just build it ourselves.”
Why they say it. Engineering led teams overestimate the v1 and underestimate the maintenance, and building feels safer because it stays under their control. Sometimes it is a polite way of saying they do not see enough value to pay.
Do not: Do not insult their engineering team or claim they cannot build it, which turns a buying decision into a pride contest you will lose.
You absolutely could build it, your team is clearly capable, that is never the real question. The question is whether you want to own it forever, the v1 is the cheap part and the maintenance, edge cases, and roadmap are where it quietly eats a headcount. What is the actual cost to your team of carrying this themselves for the next three years instead of shipping your core product?
Then ask. If you build it, who owns it at 2am when it breaks, and is that the best use of that person?