I suppose this depends on whether you see the Mass as being a) primarily people-centered, or b) primarily God-centered.
If you see the Mass as something that is primarily for the people, I can imagine why having an only-vernacular Mass would make sense. “The Mass, after all, is about us—we are the people of God, etc. etc.” This mentality leads to abuses of the liturgy, such as hosts of EMHCs (“all the better to foster our ‘active participation’”), impositions of the priest’s personality onto the Mass (“Mass, after all, ought to be entertaining; so why shouldn’t Father dress up like a clown? It makes the children laugh”), and non-liturgical music (“I want Somewhere Over the Rainbow played at my funeral because it’s my favorite song”).
But if you see Mass as being primarily God-centered, then you realize that it not, first and foremost, about you. The Church prays in Latin because she wants to be certain that her praise to God is exactly what she means it to be. Moreover, it sends two messages: first of all, that of continuity, both of expression and of visible form, with the Church’s history, and second, that the Mass does not, in fact, depend on your understanding. Even in English, the Mass is a mystery. The problem with the vernacular is that people believe that when a thing is immediately comprehensible them them, that they’ve got it “under their belt”. That is not the case with Holy Mass, and people ought to be reminded of that.
I’m not saying here that the considerations of the people are completely irrelevant to the way Mass should be celebrated, but rather, that a certain hierarchy of goods must be respected.