I purchased both, mostly because I wanted to take advantage of my new 4k TV in 2017. I do think both were needed, as the PS4 and Xbox One were underpowered from the get go. I do have to echo other users here though, the One X is one of the top consoles I have ever owned in terms of design, sound, and the upgrade it brought. The PS4 Pro was okay, but it wasn't nearly a big enough leap graphically, and mine has become insanely noisy so I don't appreciate it quite as much.
This gen both Sony and Microsoft went for pretty cutting edge hardware, so I can't see a mid gen refresh being needed as much. On the other hand, since Microsoft is taking the PC lite approach, I could see it happening anyway as soon as Graphics cards have seen major improvements