The WS5 headphones will be primarlily options when buying a Deus . . . . you can buy it with the WS4 backphones or the WS5 headphones. No one has a gun to your head to buy a pair when they come out if you have a Deus already . . . . simply another option that is available.
As Jerry said, there are benefits that the WS5 offer that may clearly justify the price.
And before everyone starts bemoaning the cost, the list price on the WS4 headphones as a stand-alone item here in the US is $452 - not sure what people think the cost is on those but since it offers much of the capabilities of the control unit, they are not cheap either..
There are options that can be used - both homemade and commercially available - if you have the WS4 headphones but for those buying a Deus or wanting the larger screen and the other benefits provided by the WS5's, there will be a choice.
Having used the two WS4 over-the-head options as well as the WS5 headphones, each has its benefits and offer great performance whcih is what it comes down to.
Andy