The concern is that if lots of softwares get rewritten and some of those softwares switch from a copyleft license to a permissive license, then things might stop being open-source sooner or later, because companies are not anymore forced to open-source.
Yes, in the case of sudo-rs, this concern is silly. But for example, the uutils coreutils are under MIT license, when the GNU coreutils were under GPL-3.0.