I don't understand why it has to be either the Novus Ordo or the Old Mass. Why can't it be both? After all, that is what the universal indult would do: it would permit both forms of the rite to co-exist.
No one is arguing for an either or. What is being stated is that the Mass of St. Thomas Aquinas--the Pian Mass--was never decreed null and void. Therefore, there is no reason why an "indult" was needed to say it to begin with.
The Church has had many rites for centuries with the Ambrosian Rite and others among those being featured.