There were priests of the Etruscan tradition, and their constituents, begging to be allowed to perform a ceremony to drive away Alaric's army in 408. Those are some pretty old gods.
In fact almost all of the "foreign cults" were just priestly colleges or lines of initiation from places the Empire absorbed. Magna Mater, Orphism, etc, are not really "less traditional" than the public political cult. Nobody was disbelieving in Capitoline Jove and Juno, there was just no initiation to make the political gods relevant to your own fate and afterlife. There wasn't supposed to be, because these were different spheres of spiritual life. And the "cope philosophies" like stoicism and epicureanism were basically just hobbies for the senatorial class. None of them advocated abstaining from public cultus. They were also mostly dead by 300, and from there the competition is between platonists, who were the most traditional, the gnostics, who were hysterical effeminate easterners, and the christians, who were hysterical effeminate easterners who desperately stole whatever they could from both the platonists and the gnostics.
Mithraism actually was fairly novel. But it's still retarded to imply that it was some kind of degenerate antisocial force. Mithraism was an incredibly effective lodge network that held the army and the administrative castes together. Have you noticed how much shabbier and stupider America has gotten since Masonry fell out of the saddle?
As for Julian, I have read every single surviving line of text he ever wrote, down to the most banal letters. As much as I might want him to be a great reformer and innovator, the fact is that all he did was reinstitute public sacrifice and instruct public priests to hold a higher ethical standard. He wasn't popular in Antioch but that's because he was a brawny illyrian goat in a city where everyone else is a perfumed syrian twink. His Gallic soldiers adored him. The entire traditionalist half of the empire adored him. When he died, Libanius reports that citizens who prayed to him had their prayers answered.
If a child was about 10 during his reign, and saw the hope he brought to Gaul and Thrace and Anatolia as he passed through, he would have lived long enough to see the half of the empire that loved him be completely overrun and destroyed while hapless schemers and brutes like theodosius tyrannized a sinking ship from behind the massive walls of constantinople. That's how quickly the fall happened after Julian lost, one single lifetime.
You have to realize that Roman paganism was a shitshow by the time Christianity gained traction. Nobody believed in the old gods anymore. The whole thing was a mish-mash of foreign cults and cope philosophies. Just look at Julian's disastrous attempts to reform it. This was not Numa's Rome, nor were these the Romans whose piety had astonished Polybius. Nothing of value was lost when what remained was washed away by Christianity.