Philip, it's hard to keep people in for the long haul when the industry is telling them that the sailing they do and the gear they do it on is old-fashioned and slow and should be thrown away, and replaced by something that won't normally work as well when they head down to the beach. I'm sure that now the sport has moved to widestyle boards that plane early, we'll agree that the '80s and '90s gear people were levered into after the sport's boomtime was NOT well suited to the conditions most of us sail in. Don't blame the gear that created the boomtime for the collapse that may have been caused by the gear people bought AFTER the boomtime (sinkers for lake sailors, DSBs they couldn't gybe, etc).
On the average day in most places in the world, the really old stuff is often better performing than the new stuff.
I don't know what's happening in skiing but I'm not sure the MTB analogy is right. The modern MTB seems to be at least as tough as the originals; modern boards aren't. The modern MTB goes better on the average day than the originals; modern boards work better only when there's a fair amount of steady wind (unless you want to drag a big sail around, and even then you still need a fair whack of wind to be faster around all angles on the typical waterway). Modern MTBs are easier to own and use than the originals; I'm not sure that a modern board is easier to own and use for most of a typical day on a Bavarian or Parisian lake, Queen Mary reservoir or outside of Des Moines or Sydney and I'm positive it's often slower.
The development of modern windsurfers seems to be a bit more like the development of specialist downhill MTBs; great for certain specialised conditions, but not for normal use on t the average day for the average rider in the average area. It's great stuff, but perhaps also a recipe to create a fringe sport.
Also note that neither dinghy sailing or surfing, the two sports that created our own, have developed dramatically in technology in terms of what the average user owns. If anything, surfing has moved to longer, cheaper pop out boards. Yet neither surfing or dinghy sailing has suffered a drop like windsurfing. And if you blame old gear for the fall off in the numbers who windsurf, then how do you explain the fact that the sport is still pretty small?
I'm not having a go at the gear per se, but more the fact that arguably it is aimed at a narrower spectrum of sailors, areas and conditions. Other sports have widened the available choices (MTBing itself was a widened choice of bike riding, surfing has gone from about one style of board to many, dinghy sailing has opened itself up at each end of the spectrum, etc)
Windsurfing alone of these sports has concentrated on one aspect (planing ie medium/strong wind) performance and windsurfing alone has shrunk dramatically in recent decades......