Firstly, "they" for singular-gender-unspecified has a long historical usage in English (https://public.oed.com/blog/a-brief-history-of-singular-they/).

Secondly, we need a gender-nonspecifying term anyway for people to avoid the need for "he or she" etc. which can really clobber sentence structure. We used to have "he/him means unspecified", which was also stupid, because when it was supposed to be specified you couldn't always tell. The singular/plural confusion is less problematic than the specified/not, since number is a lot better specified by context than gender. "When placing your baby in BouncyJoy Baby Bouncer (TM), please ensure that their feet reach the floor."

Bringing back singular gender-unspecified "they" to serve this role is parsimonious.