This ignores my point that historically Catholics learned basic prayers in Latin and did a lot of their praying in Latin.

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I have heard this claimed often, but I am not convinced it is the case.  The only evidence I've ever seen was in a movie.  I know that many saints, even lay saints like St. Thomas More, prayed in Latin.  But in that case there was a high level of learning and education, and they didn't just know "how to say prayers" in Latin, they knew Latin.
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Latin is more efficacious. Just like having the Mass be in Latin lets us know this is not an everyday activity, if you pray in Latin it may help you concentrate more and know you are not talking to your friend but to God.

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Latin is more efficacious for practical reasons, not intrinsic ones.  As a dead language it ensures constancy of expression, and therefore (and much more importantly) constancy of meaning for an institution which is responsible for safeguarding and explaining the truths and concepts of the Catholic faith.  But there is nothing intrinsically more worthy about the phonemics of Latin or any other language. 
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I would agree of course that one of the effects of Latin is its 'other-worldly' character, but that is a character that exists precisely because people are not native speakers of it.  If we spoke and read fluent Latin, that effect would diminish.
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If Latin had the effect of enabling a person to concentrate more and to know that they are talking to God rather than a friend, I think that would be a subjective rather than objective difference.  If there's evidence that I'm wrong, I'll happily acknowledge it, but I think that I'm not unique in the following: meditative prayer (which is what the rosary is) is more easily meditated when one is not switching between languages, as one would be doing if they prayed in Latin (unless of course they could meditate in Latin, which they probably can't). 
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I am obviously not against people following along in the Missal. But again, if you go to Mass ever Sunday you will likely understand much of the Common in Latin

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What you will develop an understanding of is what Latin words signal English words.  You will not 'learn Latin' and you will likely not understand Latin in any meaningful sense of the word.  Let me give you an example.  I have a four year old who can pray the Gloria Patri in Latin (we pray our rosaries in English mostly, but our Glory be's in Latin).  I didn't even know she could do it (until I heard he say it once), because I never taught it to her.  She just picked it up by virtue of being around.  But she doesn't know what any of it means.  Not only does she not know that "Gloria Patri" translates to the English "Glory be to the Father," she has absolutely no understanding of the syntactical rules of the language, either.  She is simply repeating what she has heard through habituation.  Heck, she doesn't even understand that she is speaking a language that is not her native one. 
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Now, it's a little different with adults because our cognitive functioning is quite a bit better than a small child's.  And a very short prayer, like the Gloria Patri, might be understood the same way that it is in English, or at least close.  But without actually learning the rules of the Latin language and without developing a Latin vocabulary, you can't think in Latin, which means that the best you can hope to do most of the time is see a Latin word and identify its English cognate based on experience.  Are you familiar with Searle's Chinese Room?  That's basically what we're talking about here.  When it comes to understanding a person needs to know more than just how cognates exchange, which is essentially all that (I would maintain) most English-speaking trads 'know' doing when they see some Latin that they 'understand.' 
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Don't forget that the modern Latin spoken and used by the Church is pronounced  differently, as inroads have been made by modern Italian.   So modern day "church Latin as my Cambridge-educated high school Jesuit Latin teacher insisted,  frequently would have sounded different from first century Latin.

This ignores my point that historically Catholics learned basic prayers in Latin and did a lot of their praying in Latin. Latin is more efficacious. Just like having the Mass be in Latin lets us know this is not an everyday activity, if you pray in Latin it may help you concentrate more and know you are not talking to your friend but to God. I am obviously not against people following along in the Missal. But again, if you go to Mass ever Sunday you will likely understand much of the Common in Latin

Praying in vernacular has existed in the Church since the beginning, since prayer is spontaneous as well as in set form. Latin isn't necessary (I wonder if the early Chrisitna catechumen were instructed the Our Father in Latin?) I personally don't understand the traditionalist preoccupation with Latin . It's venerable, yes, but to me its not a big deal.
Here is an interesting book, Prayer In Peasant Communities, which examines the prayer life of medieval Catholic Sweden: https://gupea.ub.gu.se/bitstream/2077/22489/9/gupea_2077_22489_9.pdf

According to several synodal statutes from the ecclesiastical province of Uppsala, written in the fourteenth and fifteenth centuries, the Pater-noster, Hail Mary and the Apostolic Creed were required to be translat-ed (into the vernacular languages Swedish and Finnish) and taught by all parish priests, and several translations remain, although only in Swedish...
According to a synodal statute from the year 1492, made by Bishop Magnus Stjernkors of Turku (Åbo), written translations of the prayers Paternoster, Hail Mary and Creed were to be available in all parish churches, and the priests had to teach the parishioners these translated versions of the said prayers...
The Paternoster was a prayer intended to be prayed in the vernacular by the laity, and the abovementioned statutes and vernacular transla-tions of the prayer indicate strongly that the vernacular use of the Pa-ternoster was an important part of the prayer ideal for those belonging to peasant communities. (pgs 115-117

Was the peasantry to use Latin in their prayer, even if they did not understand what they were praying? Siælinna thrøst concludes that the understanding of a prayer is most important in order to gain efficien-cy – one should pray in a language that one understands. Although a prayer keeps its power regardless of the person saying it, it can how-ever be used more efficiently and forcefully when said by a person who knows the language of the prayer. Prayers should, therefore, be said in a language understood by the praying person, and the prime example of this interpretation according to Siælinna thrøst, is Jesus Christ who taught the Jєωs the Paternoster in their own language.82 The clergy un-derstood Latin, as Siælinna thrøst describes it, and should therefore pray in Latin. The laity who did not understand Latin should pray in the vernacular instead, although it can be assumed that at least some of the laity did know commonly used Latin phrases and prayers, simply through osmosis. Therefore, it was not necessary, or required, to learn prayers in Latin without understanding them. Siælinna thrøst concludes that the relation between Latin as language for prayer is a matter of understanding, and makes, for example, the parallel to prayer books, which can be held by a child and by a master, but it is in the hands of the master who understands the words that they become more useful. Therefore, a priest educated in Latin should pray in Latin, and an un-educated person should pray in the language known to him or her. It is, however, not punishable for laypeople, states Siælinna thrøst, to say the Paternoster in Latin, if it is said to honour God  (pg79)