Interesting. I have noticed something very similar, within my own parish. Four years ago, we had much more happy-clappy/tambourine/guitar/folk-type music, but then we added a second musical director and that started to change. It’s been a gradual change, initially just once or twice a month, we would have more Latin, more chant, etc. As of lately, our choir utilizes the choir loft, more often than not, sings mostly in Latin, even if we sing a few verses in English, the tambourines and guitars have been replaced with a cello, a violin and an occasional harpist…but the other music hasn’t been done away with- it simply isn’t the norm in my parish anymore. It’s been such a gradual process, there don’t seem to be many criticisms of it, even though a fair number of our older parish members were initially uncomfortable with the idea of “reverting”. I think, within my parish, both needs or preferences are being met, and I’m okay with that, even though I have a hard time feeling reverent during the tambourine/guitar masses. ( It isn’t really about me- something I have to remind myself of, lol!)
At any rate- I think you are correct in saying that a gradual moving to more traditional music is effective in some parishes. It is working in mine.
Why do you see the tambourine and guitar as “secular,” but the cello, violin, and harp as “sacred”?
For someone raised around mainly classical music and concert halls, the cello, violin, and harp are every bit as “secular” as the tambourine and guitar! In fact, considering the current rampant popularity of harp music at social gatherings, especially “wine and cheese-type” parties and wedding receptions, I would say that the harp is becoming the “electric guitar” of the times!
As for tambourines, they are at least mentioned in the BIble several times. You won’t find mention of violins and cellos! BTW, have you seen the extremely pretty and curvacious violinist who plays with the Celtic Women? Wow, that is one gorgeous girl, and she is definitely NOT using that violin for sacred music! In fact, I wonder that after watching her, how any male can watch anyone play the violin without getting “itchy.”
If you are into classical music, this is definitely the trend in classical music nowadays–really “hot”-looking young, in shape, men and women dressed quite seductively and moving around suggestively while they play their arias and concertos–so does this kind of secularity disqualify the various classical instruments from use in Mass music, just like rock music disqualifies the guitar as a Mass instrument?
And how about the “fiddles” used in country and bluegrass music? These “fiddles” are just “violins” by a fancier name. It’s still “secular” music, and people who are into country and bluegrass music will associate that Mass violin with their secular music. So it seems to me that the violin is definitely on the “questionable” side when it comes to its use in the Mass. Pretty scandalous!
Do you understand what I’m trying to say here?

ANY instrument can seem secular or sacred, depending on what people have been accustomed to in their daily lives. An instrument becomes sacred when the Church and Christian people “reclaim” it back from its secular uses, just like we become “redeemed” when we are baptized. In this day and age of history, there is no such thing as a strictly “secular” instrument.