The Rambam, Ramban and Ibn Ezra certainly did not speak what is known today as Ivrit.
They probably spoke something close to the Judeo-Arabic that Moroccan Jews spoke until recent generations. (I think Ladino came later but that is another possibility.) That was their “Yiddish.”
Our ancestors all spoke Aramaic as a common tongue at one time. That would be the best solution – renew it as a daily language among frum Yidden, keep Yiddish as long as people want to keep it, and make it clear that Ivrit has no kedusha and should be considered as a last resort for shiurim, even below the local language.