Here in Bermuda is a small reality but it make sense to the discussion.
You can have only one car per family but as many scooters as you want.
When I moved here the plan was to learn Windsurf but I had two major constraints:
1) Buy a decent car with a rack
2) Get the rigs delivered on the island
So I decided to learn Kite. With less than 2,000 USD I bought a brand new North kite, with bar, board and whatever else.
As soon as I got 3/4,000 spare, a car and the time to learn, I moved to Windsurf.
Honestly, I would never, ever move back kiting ...

So, in order to answer your question: windsurfing is not declining but it is a lot more expensive than kiting and it requires more stuff (i.e. a car, more money) than kiting ...
My two cents