I think hardware arrangers will be around for a long time. The controls are (obviously) tailored to the needs of the arranger player, and the sounds can only get so much better before the law of diminishing returns sets in. These days it makes sense to use an arranger with a computer to get the best of each (convenient access to styles and the best sounds). In the future, there will be less to gain by adding the computer.
Don't foget, computers are "patch-work" instruments: different manufacturers for each computer component and each software program. They don't always work together as well as you would like. If you are into composition or electronica, computers are the way to go, but if you are into entertaining a broad segment ofthe population, I think hardware arrangers are the way to go.