While I agree that there are problems with the Liturgy as it is currently celebrated, I hesitate to throw the baby out with the bathwater. "I am the Bread of Life" is a beautiful hymn, guitars and all, and I think it serves the same purpose, in its folky way, that the music of, say, Palestrina did in his day -- listening to the "Agnus Dei" in Palestrina's "Missa Brevis", it is clear that his purpose is to lift the congregation's mind and heart to God, and to plaintivly ask for mercy in a way that makes it clear that God in His goodness stands a good chance of granting it. "I am the Bread of Life" attempts to do the same thing, in the sense that it is a reflection on the promises of Christ (though I grant there is quite a distance between the artistic merits of Toomey and Palestrina).
I think there ought to be a pluralism of music in the Mass, and in fact my home parish does this fairly well. In one of the morning masses, there is more traditional music and the Sanctus and Agnus Dei in latin (and often sung in chant). The youth Mass is the one where you're more likely to hear guitars and Marty Haugen. This seems to work well, and there have been few complaints.
There are certainly limits to this that have to do with taste -- I for one will avoid the "hip hop Mass" if it ever comes -- but it is worth remembering that Christ wasn't too concerned about form in worship, and in fact the Institution of the Eucharist happened in an intimate and informal (though solemn) setting.