"Wear" is exactly right; most toasters have a pretty crude design involving hot glowing wires, springs and an electromagnet that jolts the whole thing pretty violently when it's finished. Just the glowing wires will fail after a while, even if you make them extra thick.

(That said we have a basic model secondhand toaster that gets used multiple times a week for the past six years and it's still fine... although some of the glow wires don't work).