It feels like the author is keeping the slime around because it was the only one who expressed honest interest in how speedrunning works, and MC can't help but want to help others get into speedrunning. Unlike the princess and knight, who are terrified of speedrunning, the slime is a blank slate that wants to understand it, and MC probably can't help but see a prospective student to teach (or learn from, a fresh perspective), like that girl in an earlier chapter who accidentally discovered wall-phasing and that old man who asked to become a disciple.