Why does spaghetti turn sticky and limp when it keeps boiling past al dente?

Answer: Starch swells and leaks

Gluten becomes stronger — Not quite. Gluten gives pasta its springy backbone, but extra boiling does not keep strengthening it like a tightening cable. Once water keeps moving in, swollen starch and a weakened structure take over the bite, which is why overcooked spaghetti feels slack even though wheat protein is still there.

Salt pulls water out — No. Salted water mainly seasons the pasta, and the salt effects on boiling point and texture are secondary. That does not explain why a noodle goes limp after too long in hot water. The limpness comes from too much water getting in and starch escaping, not from salt pulling water out.

Starch swells and leaks ✓ — Right. Heat and water make starch granules hydrate, swell, and gelatinize; pushed too far, more starch escapes into the water and onto the surface. That makes noodles sticky while the inner network loses its spring. The same starch later helps sauce cling, but once it leaves the pasta it leaves the noodle slack.