It’s like saying you’ll rewrite Romeo and Juliet because you already know they die at the end. It’s a little more nuanced than that!
I have a particular stereotypical programmer in mind. The one that rewrites their entire program in X, because it's fast. Not because they understand the data dependencies and run-time performance characteristics of their program.
Typically these folks misattribute the performance gains they experience in such projects to the language itself rather than the tacit knowledge they have of the original program.