I would have expected a change to 15 mm would occur with Boost hubs instead of the 20 mm boost standard coming out. But it's hard to see how that would get sold. You can't sell stiffness, it's hard to sell weight to DH guys (the way I see it at least), in the rear going 135, 142 or 148 gives you supposed width advantage over the 150/157 hubs (narrower frame, 'harder to snag rocks'), etc.
Interestingly, 15 mm hubs should be more durable though. With bicycles using disc brakes you are limited with how big of an OD bearing you can put in the non-drive side of the hub. With a 15 mm axle, you get some more space for the bearing on the ID. That means larger balls and, in practice (considering the load carrying ability specification of said bearings), more strength. And that means more durability.
Ah, the details 