People, really, you gotta stop with the absolutes. If you have a 4K TV and are even remotely interested in watching UHD content, then of course an UHD Bluray is a great thing to have. Yeah sure you can stream, but if you want to have the physical media and enjoy more the quality of the physical disc it really is without a doubt a benefit to have a UHD player. So saying it is irrelevant, even if sales haven't picked up, I really haven't kept up with it, is all in the eye of the beholder.
Now, if you don't have a 4K TV, or you do and you are just not interested in any way in buying UHD movies and dont even care to have or not have the option, then yeah sure, why would you care? Personally I have an Xbox One S, I am glad I dont have to buy a standalone player to have the option of playing UHD Blurays, that is value. And I have a PS4 Pro where I game at checkerboard 4K and I love it, and I dont miss the UHD player since I already have one in the One S.
In conclusion, why this talk about it being useless to have it, or Sony messed up in not putting one? The Xbox One S has some great added value with its 4K Bluray, for those that are interested. And the PS4 Pro not having one is a none issue for those that are not interested. It is value, having it is a plus, it is a good thing, but not everybody is interested, I can live with that. So I can understand the disappointment in some for PS4 not having one, and the apathy of others towards the Xbox One S having one.