I think misinterpretations of Church guidelines (yet again) are one of the main reasons why much great music has been lost. I know that in Ireland at least many choirs which had specialised in chant and polyphony fizzled out after Vatican II, with many priests using council documents as an excuse - though I don’t think anyone has ever found the one which says that churches have to abandon their great choral traditions! I know that in other countries even cathedrals ripped out their pipe-organs in the wake of Vatican II - again something which was uncalled for as the Church constantly teaches that the organ is to be held in the highest esteem. I would say, as an organist, that the demise of good hymn-singing went somewhat hand in hand with the destruction of many fine church organs. Also, in the wake of Pope Leo’s motu proprio in 1903 which re-emphasised the central role of chant in the liturgy, many new churches were built without pipe-organs as it isn’t necessary to accompany chant on the organ - so I think in some ways the problems with quality hymnody in church go back further than Vatican II.
As for church-music composers themselves - there are certainly some good ones out there, but as far as I can tell at the moment, they are in the minority. The attitude seems to be one of “we have to get with the times”, but to hell with the sense of the sacred. We get “Sister Act” style music which, if you recall from the movie, did indeed fill the church, but the people were acting like they were at a rock concert. I fail to see why one would go to church to experience something divine but yet be content to listen to exactly the same style of music that they would hear in their local public house. Too much church music today is simply not fit for it’s purpose - and the primary purpose of sacred music is not to keep the people happy.
Now, to follow that rant - it’s not all doom and gloom. I’ve been associated with many cathedrals and churches as a chorister and an organist and thankfully I’ve generally experienced good quality polyphony, chant and strong hymns. I have access to a good range of hymnals, but am not too familiar with the ones used in the U.S. (though I have heard several people criticising them on C.A.F.). There are many good quality hymnals available, but I think churches and their choir directors are often content to simply use the most popular one, which in my experience is rarely the best! Personally, I always choose hyms very carefully - not just according to the season or feastday, but also to the meter. Meter is very important and is the key to a good strong hymn. A steady meter gives direction to the hymn and it helps the people to sing with gusto. Hymns which merely meander and lack definite meter tend to drag and become dull, and few people have the confidence to really sing out in case others begin to take liberties with the rhythm! To answer your last two questions: I am one organist who has not forgotten our great traditional hymns and I am always searching through old hymnals to find unknown treasures. I’ll do my best to ensure that they aren’t lost!!