Arranger decline:
In the 70s and 80s the hone entertainment organ was king, with a boatload of different manufactures from all over the world, however by the 90s the number of manufactures had diminished with the number of models on the market being significantly reduced, those remaining manufactures continuing through the 00s and 10s, but come the 20s only 3 remain (Wersi, Bohm & Ringway (Although if you don't mind importing from Japan you can still get a Yamaha) which are ticking along very nicely in what is now a niche market.
The Arranger keyboard took over from the hone entertainment organ in the 90s, with a boatload of different manufacture out there, these became thinned out in the 00s with only the big boys remaining, which soldiered on into the 10s, however by the 20s only 3 where really doing anything substantial (Yamaha, Korg & Ketron) in what has now become the final stages before they become niche.
So yes, the arranger keyboard is dying, as people move on to other things. (The phone and iPad are the in things these days)
Although not technically arranger keyboards (There actually organs with arrange capabilities) both Bohm & Wersi make a single keyboard version of their organs, which rather than being regularly replaced are constantly upgraded with new hardware and software that works out a lot cheaper. (Also, as they are organ first, then if you add pedals and a second keyboard you have a full organ that operates exactly the same as the larger organs)
Wersi & Bohm also have comprehensive Midi specs (Something lacking in most arranger keyboards) so that you can add the latest arranger keyboard (With all its bells and whistles) to give you another keyboard that operates as if they are one instrument.
One more thing to take into account, is that most arranger keyboard players are more mature and will not be around forever, thus the market will dwindle even further.
It's sad, buts that life.

Bill