Ora,

Thank you for pointing out that there are different antiphons sung/said.

Why?

The compilers seem to have regarded it as a duty imposed by "The treasures of the bible are to be opened up more lavishly," SC§51. In this case Nunc gaudeo in passionibus pro vobis et adimpleo, ea quae desunt passionum Christi in carne mea pro corpore eius, quod est ecclesia, (Col. 1:24 NOV). For singing in Latin, of course, the treasury of Gregorian chant had to be preserved.

Ora,

Do you mean that the people who assembled (cobbled?) the new rite intended it to be spoken, only to realize that the word "dicit" and its derivatives in the old Missal always implies "cantat", and hoped people wouldn't notice?

I think the new Office was clearly intended for busy diocesan clergy and also to be easily within reach of the laity. The pre-Conciliar Office had a corpus of Gregorian chant for it, but the reality is that most clergy recited it so the new one was designed with that in mind. The monastics of course had the Benedictine Office which they continued to use after the Council, plus a couple of other specific post-Conciliar monastic schemas spread over one (schema B) or two (schemas C and D) weeks and that use the antiphons from the Antiphonale Monasticum and Psalterium Monasticum. They probably figured that this would continue in monasteries so they didn't need a "singing" Office. I should re-read the parts of Bugnini's book about this (I have a copy). I read it a couple of years ago.

However I'm not sure what they had in mind for other non-Benedictine/Cistercian/Carthusian orders and congregations that did sing the Office. Most who retained chant ended up putting something together, in binders, using antiphons from the Monastic Office or with vernacular music such as from "Chanter l'Office". Some psalms of course had new divisions and were missing an antiphon for the second part. However discretely buried in the General Instruction was the option to say both sections as a the single psalm that it is, under one antiphon. In some cases the Monastic Office gives two antiphon options, so they'd use the first for the first part of the psalm, and the second for the second part. It was all a bit bric-à-brac, with loose-leaf binders and photocopies of photocopies...

Now, with the first two volumes of the Antiphonale Romanum, the Liber Hymnarius, and Les Heures Grégoriennes (which follows the OCO for at least the ferial Office), things have improved considerably and it is now possible to sing the entire Office in Latin gregorian chant from books in print with the exception of the Office of Readings. I used the OOR as Matins in my own observance of the hours, early in the morning. I put together my own Nocturnale for it using the earlier edition of the OCO, now made obsolete by the 2015 edition. But I prefer simply singing it recto-tono except for the hymn as is done in the abbey I'm associated with. Or did. Madame l'Abbesse has decided "no chant in the house before 8 am" as she's a late riser and a light sleeper. Now that she's retired, she likes to sleep in. So I read that Office in bed at around 5:30 am. In summer, the window is open, and the birds provide the music for me! They sing far better than I can! At least some species. Blue jays... not so much.

IMHO what the reform of the liturgy should have done was have the LOTH as it is for private recitation, and some sort of meatier office, perhaps on 2 weeks instead of 4, with music for chapters of Canons, non-monastic orders and congregations, etc. There is a schema in the LOTH to do this for the Office of Readings (Vigils). For my purposes, when I get sick of the LOTH (especially Vespers of week III where the mid-day "minor hour" has more psalm verses than Vespers, a "major" hour; I call Vespers of week III "Vesperettes"!!!), I fall back on the monastic schema of my abbey but on a 2-week variation that is described in the rubrics.

Ora

CGZ - to commment on both your questions -
2) the same factors as caused founders of communities of nuns generally to opt not to impose the full Breviary on their communities.
1) The importance of the the sung antiphons and responsaries to communal celebration was emphasised (judging by Bugnini's account; Reform p551ff). But they were optional in private recitation, the reformers thought them valuable for fostering prayer, but could not always find sources in the 'treasury' which matched the text to the feast, so included alternative scriptural quotations to be spoken. Research has found other forgotten sources, hence the updates to OCO.

Yes I remember reading that in "From Breviary to Liturgy of the Hours".

@CGZ:

but what do you think might have made (in the minds of the reformers, anyway) the older form of the Breviary somehow inacccessible to ordinary lay Catholics?

The two Ls: Latin and Length. Even secular clerics involved in parish work found it too long. Of course the Vatican could have removed one L by allowing approved vernacular translations. If one needs to look at the vernacular translation while praying in Latin which one doesn't completely understand, it really slows things down. In the current LOTH, I can chant the major hours (Lauds and Vespers) in Latin, with a silent pause to read the psalm in French (my vernacular) in about 20-25 minutes on most days, (except for the very short Vesperettes of week III which I can do in about 15-17 minutes). Or I can chant the monastic schema of our abbey in French, which is 4 psalms plus OT canticle at Lauds and 4 at Vespers (with at least one long psalm at Vespers, such as 144), in the same time. If I do the monastic schema in Latin (as I do on Sundays when I do that schema), it's 30 minutes instead of 20.

I'm retired and I still sometimes have to skip an hour due to some unanticipated toil sprung on to me by la Mère Abbesse... but since I'm not bound to the Office it's not an issue. As oblate I only promise to pray as much of the Office as I can without... upsetting la Mère Abbesse... ;) For priests bound to the Office with sick calls, preparing the next homily, funerals, counselling parishioners, giving last rites, daily Mass, and not to mention parish administration, it can probably be quite a struggle. Or so I'm told!

Ora

A system that worked well for the vast majority of Church history

Is there supporting evidence for this claim? It was not not the view of Pope Clement VII in 1535, nor of all his successors. I have seen documented complaints from laity that they would turn up for Vespers or other Hours and find no clergy, since they were all in the tavern spending their stipends after each had said a perfunctory chantry Mass

I have seen evidence of Vespers and/or Compline attracting congregations in the past, at least on greater feasts. But there were also plenty of complaints from secular clergy, and from bishops, about the daily grind. And these were strongly expressed again at VII.
There is tension between Monastic Office and Cathedral Office, with Catholic Cathedrals over the centuries opting either for actual monasteries, or College of Chaplains which offer similar service without the vows of stability (or poverty!).
The Consilium failed to resolve this tension, I doubt whether a committee could - they produced LOTH. Maybe a Pope with the courage to select a Quiñones, and license him to conduct diocese sized experiments over decades! failing that I see no possibilty of enduring change.
CGZ - yes, but remember the congregation was taxed to provide the service, and would rightly complain if it were not provided even if few were eager consumers of the service.

Stick to the topic, please.