Saying you have no intention of doing something then doing it is not engineering, it's being dishonest. He could have said "well decide when we see the results", why didn't he?
I'm guessing that if I said it ... that we have no intention of re-writing in rust ... that what I mean is "we have no intention of spending the extreme cost it would take to rewrite". When I discover the cost model is completely different that changes things.
If you mean "we have no intention of spending the extreme cost it would take to rewrite" then say that, and it would be fine. If you instead say "we have no intention of re-writing in rust" you've said something very different, using a different set of words, which changes the meaning. Especially, if you say it directly in response to someone asking you whether you're going to rewrite or not like was the case here, and say that there's a high chance you'll just be throwing it away, to get the other person off your back. If then you go ahead and do it, expect them to call you out for it.
This is a very simple concept that can generally be understood by children at around age 4. Trying to cover it with vague terms and using the defence of "well I said I had no intention, and I probably won't do it but you see, I saw the results so I changed my mind so the chance was small but not zero", that's what a slightly older kid will try to do to see if they can get away with it, and as any kid discovers, that doesn't fly.