Manufactures are well aware of the changing times and will adapt their boards to suit, however only when it becomes profitable to do so. (Making money for their shareholders is their main concern)
The big 3 could quite easily make a Wersi OAS or an MS instrument today, but little money would be made, as the big bucks are made by selling new boards. (Even though they may be only marginal upgrades, folks always want the latest)
As an example take the Tyros range, it has been in production almost as long as Wersi OAS, and most users have been happy to upgrade (And loose a fortune in the process) to T2 then T3, whereas a Wersi owner has just upgraded the software/hardware as required, and so has cost them a fraction of the cost of Tyros owners. (The big boys will always go for the biggest profit, and is why they have not yet gone the Wersi OAS route)
Interestingly Yamaha has now started along the option route (Buy the options you need (Premium Packs) just as Wersi have been doing for years, so it looks as if at least one of the big boys is now slowly changing. (I still think there will probably be a T4, but after that I think they will go the whole hog for the modular format, as people will be no longer prepared to loose a fortune of their cash, just to get a small upgrade)
So Basically
Arrangers will continue, just not in the current format.

Bill