Jocelyn Wang replies to me:

>...  Children have voices that are uniquely child-like, nor are they
>generally paid for being in the choir.  Likewise, the same justification
>can be made for choirs that are exclusively of one gender, paid or not.
>The sexes have distinctly different voices and therefore sing differently.

I mostly agree with your analysis, except for the part about children's
voices being uniquely child-like.  You can train adult trebles (male and
female) to sing with that sound.  In fact, it's done all the time.

Steve Schwartz