Here’s an anecdote, related, in sort of the opposite direction...or at least more complex. I don’t really like TOS. It’s not my thing. Family members know I like Star Trek however, so I still own TOS merchandise, because they get it for me (and my son) at Xmas etc. I have heard people shopping in Forbidden Planet in the old days, and ‘well, he likes Star Trek’ is more likely to result in TOS stuff being suggested, there’s more of it, it’s seen as the safe choice. Now, that’s not much in the way of ‘tng era more popular’ but it is, I think, a part of why TOS shifts more. Anecdote too, but like I said, I think it’s nuanced. The only thing I can tell you is KT doesn’t shift toys, didn’t bother with books, but becomes the default for fancy dress costumes by virtue of being cheapest.