You have been selective in your reading of Sacrosanctum Concilium in using it to say that “the majority of the mass should be in Latin” It says no such thing. No.36 makes clear that Latin should be preserved but then clearly gives ecclesiastical territories the authority to decide how much should be preserved, subject to approval. This isn’t an “innovation by those who thought they knew better” but Bishops exercising their lawful authority, and confirmed by the Apostolic See.
It doesn’t say that Gregorian chant is the “norm”, but that it is specially suited to the Roman Liturgy and *all other things being equal *should be given pride of place. Musicam Sacram (1967) further clarifies that Gregorian Chant should have pride of place “in sung liturgical services celebrated in Latin.”
A friend noted (and Fr Z. blogged on this too) that a literal translation of Sacosanctum Concilium (Vat II) reads:
#116.The Church acknowledges Gregorian chant as characteristic to the Roman liturgy: which therefore in liturgical actions, other things being equal, should occupy the first place.
So other things being equal, i.e. in normal circumstances, GChant should have first place, not last place or no place at all. This seems to suggest is is the norm. I think it was Pope JPII or BXVI who said GChant is the the chant/song proper to the Roman Rite.
Musicam Sacram #4 (again with a literal translation) states:
b) Under the term sacred music come these:
Gregorian chant,
ancient and modern sacred polyphony, in its diverse styles,
sacred music for organ and other permitted instruments,
and the sacred music of the people, liturgical or religious.
“For the celebration of the Eucharist with the people, especially on Sundays and feast days a form of sung Mass (Missa in cantu) is to be preferred as much as possible, even several times on the same day (Musicam Sacram #27, 1967).”
The General Instruction of the Roman Missal states: 41. The main place should be given, all things being equal, to Gregorian chant, as being proper to the Roman Liturgy.
Elsewhere it says (of the Entrance Chant): 48. This chant is sung alternately by the choir and the people or similarly by a cantor and the people, or entirely by the people, or by the choir alone. It is possible to use the antiphon with its Psalm from the Graduale Romanum or the Graduale Simplex, or another chant that is suited to the sacred action, the day, or the time of year,and whose text has been approved by the Conference of Bishops.
No mention of folky/pop guitar songs.
The quote you offer from Musicam Sacram does not say Gregorian Chant holds first place only when Mass is in Latin, or that if it is in the verncualr one is exempt from chant.
As for Latin, yes the limits of the vernacular may be extended but the Council never said it can be abolished altogether. It said it must be preserved, that the faithful should be able to say or sing their bits of the Mass in Latin, and that her priceless treasure of sacred music (in Latin mostly in the Roman Rite) be fostered.