If the pope is infallible when he defines a doctrine of faith and morals, what need is there for an ecumenical council?
The fact that Vatican II made no dogmatic pronouncements — it was a "pastoral" council — seems to prove that there is no real need for an ecumenical council anymore in the modern world.
Pope Pius IX pronounced a dogma ex cathedra in 1854, then Vatican I formally ratified the pope's right to make such dogmas; and Pope Pius XII pronounced another dogma in 1950. It seems the Church has entered the age of papal autonomy.
Especially now with modern communication, if the Church needs any dogmatic definition, the pope can easily query bishops around the globe — if he wants, though there is no necessity for that either — and issue the pronouncement ex cathedra.
(One wonders if St. Peter knew he didn't need to convene the Council of Jerusalem to decide whether Christians must practice the Jewish customs; if he had decided ex cathedra, then St. Paul would not have had the opportunity to rebuke him).
Is there any situation that would really require an ecumenical council? Perhaps they have now fulfilled their purpose by developing and defining papal infallibility. Maybe they are now obsolete.
Could Vatican II be the last? Or will all future councils similarly be merely "pastoral" ?
Unless we ever get another situation like the western schism (God forbid) where there is a pope and antipopes competing for the claim to the papal throne — and given how much trouble Vatican II caused — another ecumenical council would seem not only unnecessary but unwise.