Overall, I agree with you.
The rubrics do not call for a rigid uniformity of posture for individuals (but there is certainly a call for uniformity of gestures in the general sense). At the same time, keep in mind that “A common posture, observed by all, is a sign of the unity of the assembly and its sense of community. It both expresses and fosters the inner spirit and purpose of those who take part in it” (this is from the 1974 printing of the GIRM #20–it just happens to be what’s on my desk at the moment. It’s an outdated printing, but the words still hold true).
If a few people just happen to be holding hands, in a private way, there is no problem with this. No problem at all. However, the reality is that that is not what is happening. For example, I know a married couple who simply place one hand on the back of the next pew and the other places a hand over that one. Perfectly acceptable. I repeat perfectly acceptable.
In general, and yes, I am painting with a very broad brush here-and-now, in general, what is happening in many places and among many groups of the faithful (in particular those who identify themselves as “Catholic Pentecostals” or “Charismatic Catholics”) is that the hand holding has moved well beyond the point of being merely a private, personal gesture among (let’s say for example) married couples or siblings. The reality is that the expression this gesture has taken-on is one of a corporate gesture involving the entire congregation (with a few individual exceptions). It has moved beyond being merely hand-holding and it has evolved (or devolved) into lifting the hands up for everyone to see, and culminating in a sort of “group wave” during the doxology after the Our Father. There is no doubt that the gesture has been imposed on many people who object to it; hand-holders will grab and forcibly hold the hands of neighbors with no regard to the other’s sensibilities (this is a fact, not an absolute, but it is widespread). Hand-holders will step out into the aisles and join with those on the other side.
The problem here is that many (not all, but many) young Catholics (whether young in age, or new to the Church) are being initiated into the “proper” way to behave at Mass and are being told, either in direct words and teaching, or by example, or simply as a matter of conformity, that the proper and expected gesture for the Our Father is to hold hands. And that is why it has become unacceptable, and that is why the practice has become a liturgical abuse. It would not be an abuse if it were done privately or unobtrusively by a few people. But that’s simply not the reality we’re dealing with.
The very fact that so many Catholics are discussing this matter stands as testament that it has indeed become an unwritten (and illicit) rubric in so many places, and among so many groups.