A couple comments - since I’m late to the conversation, it would be crazy for me to try to address everything that has been mentioned, so I’ll just touch on a couple.
Sure, the Church does not prohibit modern composers from composing for the Liturgy. I believe it can be effectively (and correctly, in my opinion) argued, however, that the Church does not just permit any modern composition to be composed for the Liturgy, and definitely not just any style. It still has to conform to the standards of Sacred Music which the Church has more or less upheld over the centuries.
I think the only other thing I’ll mention is this: I believe one of the best arguments for using only Gregorian Chant, polyphony/other similar works, and a small repertoire of solid, Catholic hymns, is simply this - it is specifically and obviously Catholic. Not so with modern-style music (i.e., praise and worship, Christian pop, whatever else we unfortunately commonly hear today in the Liturgy). I’m all for modern composers to compose for the Liturgy and even to incorporate modern musical thought into it - so long as it is composed in the style of the traditional sacred music of the Church.
And the reason I believe it’s best to only have authentic, undoubtedly Catholic music at Mass is to distinguish between the Catholic Church and Protestant Church/everything else. I understand the desire to be ecumenical, and one might argue that, well, we have the Eucharist and they don’t. OK, sure, that’s certainly true - but would one readily be able to recognize we have something they don’t when they walk into a Church and there’s a nice band up front complete with drums and whatnot? OK, that’s an extreme example, but in general, I would argue that more often than not, no, they would not be able to readily see, by the prevalence of this sort of music in the Liturgy as well as by how many people act inside churches today, that we as Catholics have something they don’t. I know God can work through any means, so certainly it is possible for a non-Catholic to feel the presence of God in a Catholic Church in any circumstance - but I believe the Church would be much better off if she would unashamedly sing the Liturgy in the form as close as possible to what she has tried to do over the centuries based on documents.