I understand, but suckling can get annoying if done for a long time in the absence of milk.

Example: when babies are young and there is an abundance of milk, babies can nurse for however long and it is still pleasant to the mother. With older children, it eventually gets uncomfortable because there is little milk. When my youngest kid was 4.5 years old and I had been nursing for 6.5 years straight (counting from her older sister's birth), I weaned her for this reason only - I would have been fine with her nursing for longer, as she clearly preferred the taste of my milk to that of cow's milk and was very vociferous about it, but it started to feel uncomfortable and unpleasant because I was virtually out of milk at that point. Sure, nipples are erogenous and all the rest of that, but there probably is still a limit on how much suckling motion they can sustain.