I don't think that's entirely true, (besides that Helix is certainly not that much of an experience, styled rather than themed), there are still a lot of parks adding flats across the world... Lots of Sky Flys and Air Races since they're the most popular at the current time are springing up throughout... Six Flags and Cedar Fair have mostly added flat rides recently (although SF have tried to promote them as coasters, idiots)...
There's still demand for flat rides, and it is disappointing that Merlin seem to be avoiding them... Presumably because of a lack of USP, yet Chessie would do really well to get an Air Race or generally ANYTHING...