"What about Sts. Cyril and Methodius? Pope John VIII certainly thought that the Slavonic liturgy would help spread the Gospel."
I went to a Slavonic Mass in St. Petersburg last year. All old folks. One family with children. I was the only non Slav there. Attendance about 45 or 50.
Most Catholics have never even heard of this. I am sure it spread the Faith to the Slavonic people, but it did not spread the Faith universally as the Latin rite did throughout the world.
I suspect that's not the fault of Slavonic v. Latin...