I suspect Yamaha and Korg took up the slack left by Roland's lessening product line, so both companies are selling arrangers at a good pace.
Thing is, most people who own an arranger, use barely half of what it can do...and that includes some pros.
So adding new features, irregardless of brand, is always going to be carefully thought out by the designers...remember, PROFIT is the ultimate goal for all these companies.
Ian
Also let's not forget Technics and GEM they were the big arranger giants that were copied by KORG and Yamaha and were way ahead of their time, and probably some of the programmers went to work for the existing companies. And like you said KORG,Yamaha,Ketron, WERSI are mostly left taking up most of the slack, who knows maybe Roland will make a comeback?
I wish Yamaha made a little better quality on the PSR models but then again I guess you get what you pay?