I think what happened with Trek in general is that during reruns it was watched by young kids (myself included) even though it did not target that demographic.
Most stations played it in early prime time, five nights per week. How is that not targeting that demographic?
Even though TMP got a G, I don't think it hit that sweet spot, not just because of the philosophical aspects of V'Ger, but also because the characters lost the casual familial quality that they had in TOS.
As I said, TMP wasn't aimed
at kids, but it had to allow for whole families to attend. Gene Roddenberry said as much in a Lincoln Enterprises' newsletter, IIRC.