Ever since Pro was announced, I've held to the opinion that this generation (X1, Wii U, PS4) will be as long if not longer than the last (which was approx. 7 years). Switch's success makes this more plausible since Sony and Microsoft will have to at least consider making a hybrid console in order to compete. Set-top boxes have continued to decline as the TV ceases to be the center of the home. Folks nowadays want flexibility, not power. Whether it's any-screen gaming, account sharing, "gaming hub", or some other sort of flexible way to play games, they need to offer something compelling. Paid online, a new physical medium, and/or cloud saves won't cut it next gen.
I think the Pro and X were released specifically to delay the launch of the next gen. It adds a few more years to the current platforms and gives Sony and Microsoft more time to observe the market and plan accordingly.