First time I heard about "AI-complete" and you mentioned it twice here.
Many basic concepts in computer science are in this class: data compression (see: the Hutter Prize), indexing (see: "learned indexing"), cache replacement algorithms (see: Bélády optimality theorem), etc. We narrowly specialize these algorithms such that they look very different to be computationally feasible but at the limit they are the same algorithm problem.
I find it interesting that database engines are essentially a giant bucket of algorithms all in this class.