After (fairly) long time in the industry, it seems to come down to:

- understanding what one's clients need (not what they ask for)

- understanding what's in the realm of the possible (technically, politically, organizationally)

- understanding what are the possible futures (good, great, and abject disaster)

If, after answering all those questions, one still advocating for a rewrite, you've shown the maturity needed to undertake the effort.

"Finger-gunning" is usually related to skipping those steps, not the ultimate decision.

(edit for formatting)