Well to be honest, Skateboarding has always been a fringe thing. I don't think it will ever become "retro-hip" because if that was the case this would have been that second coming. I mean don't forget about the pre-Tony Hawk days with games like "Skate Or Die" and the like in arcades... to which there were comparisons when you started to see the current round of titles.
That said I pretty much agree that skating is falling off as a fad again, and another decade before it appears in the mainstream again is to be expected.
That said, I do think that falling out of the mainstream eye is a good thing for skating in general (even if I'm not into it). From what little I know, and have seen, it seems like when skating is heavily in the mainstream eye, it has a degenerative effect on the quality of the performances, and the subculture as a whole. You see tons of poseurs swarm into it, a lot of "five minutes of fame" celebrities who get lucky just dabbling with it, and so many people aping the culture that you start to see the public passing policies (which disappear when things ebb) to limit skating.
It's ironic, but some guys I've talked to who have been into skating for a ridiculous amount of time (even if I know very little about it personally) have talked about how some of the most recent skate games pretty much summarize the problem. In those games you literally have a focus on self-promotion and pimping skate gear and fashions. A player can basically do some halfway decent, but overall unspectacular things in "competitions" and then pick up a brand and roll a skateboard in front of some photographers while they get money thown at them. The kind of stuff "Yahtzee" has made fun of, yet it perfectly emulates what skating becomes like when there is mainstream interest. That and a proliferation of people who want to heckle injuries as opposed to appreciate the sport... catered to by things like "Hall Of Meat" mode.
Yes, very little of that has much to do with Tony Hawk and his games specifically, but the point is that I tend to agree that interest in skating dying down is probably going to be a good thing for those really into it.