Digital downloads would only work if there's a good incentive to switch to it and there really isn't much of that on Nintendo platforms. They don't have a good history in-terms of how digital ownership would work, they tend to maintain their 'Nintendo tax' on digital purchases, their sales are a joke compared to steam and their online infrastructure is still questionable. Unless Nintendo provides a noticeable discount on digital games compared to physical copies on the PS4 and XB1 or gives some other kind of incentive/feature, I can't see digital-only working.