SNM:
njrick: I think there is another possible explanation, which I've heard postulated. That is that younger people are more open to experimenting with various types of sexual play, which makes it easier for them to find willing partners without having to join groups, attend clubs, or search online. It may just be one more facet of sexuality to explore, with those who find that they're into it also finding the people to play with along the way. I have no way of knowing whether this hypothesis might be true, as I tend not to hang out with a younger crowd or search for partners (of any type) in that age bracket.
I'm pretty sure you're on to something here.
If you're willing to accept anecdotal evidence, as a member of the younger generation I've encountered little to no stigma against fetishism among my peer group.
I agree with Njrick and SNM that younger people nowadays are more willing to to explore kinky interests and therefore spankos are in no danger of dying out. It is just that younger people are doing it it while older people are more likely to write about it.
Also just because something such as spanking is used less as a punishment doesn't mean that awareness of it goes away. It has been hundreds of years since swords were used in warfare and look how many films are made featuring sword fighting. Even films about the future have swords as weapons, for example, the light sabers in Star Wars.