I presume you mean "instead of each year group having a bunch of emo kids hanging out, by 20xx it was a bunch of "trans" kids." To what extent would you be able to say that the same kids that were emos would have been trans if born 10 years later, and the trans kids would vice versa have been emos if born earlier?
Were there any groups which existed at the cusp and literally changed from trans to emo?
Clearly there are always people who dress "normally", some towards the geekier or squarer end of the normal scale, and some towards the more fashionable and well dressed end. And there are always kids who choose to be part of a tribe that dresses a bit, or very, different (I was one, not an extreme version, but I was one).
I wonder how much of the teenage / social contagion end of this is nothing more than (1) the sort of kids who naturally wants to be part of an "alternative" tribe, plus (2) the death of significant cultural movements within music that would form the basis of most of those tribes? Kids subconsciously needing to be part of something different AND wanting to show that difference by the way they dress, and trans being the only outlet?