Actually, enrolments in HSC Biology significantly outnumber enrolments in HSC Mathematics Extension 1 and Mathematics Extension 2 combined. This year (2021), 11986 students are enrolled in HSC Mathematics Extension 1 and HSC Mathematics Extension 2 (consisting of 8754 students in HSC Mathematics Extension 1 and 3232 students in HSC Mathematics Extension 2), compared to 19028 students enrolled in HSC Biology.
Based on this, I believe Hivaclibtibcharkwa's proposition would be valid. In such a case, the main consideration will become content, i.e. what is the nature of the content to be taught in case "Biology Extension" becomes a reality? Of course, as mentioned above, the availability of teachers capable of teaching the extended content is still an important factor that determines the possibility of an Extension science subject, but in the case of Biology, student demand may not be an obstacle.