The question wasn't "should all humans not modify themselves", but "should humanity survive".

We currently legally protect mostly pre-contact civilizations like Sentinelese, so it stands to reason that regardless of what some people choose, other people will forgo transhumanist modifications, and "humanity" will survive regardless of what technology occurs or where your definition ends.

Unless of course the end goal is only a few billionaires get to live with their AI ppowered city states at most using servitors and decraniated as robots.