I'm going to play devils advocate here. (and yes, this is also directed at mrcorbo)
At the moment, the user install base is something like this:
PS4 ~55 million (?)
Xbox1 ~26 million (?)
I take it these numbers are more or less accurate. We know in the NA, XB1 is quite competitive with the PS4, the PS4 at a slight advantage. If we assume a 45/55 split, it means the PS4 must be leading at more than 66/33 in the rest of the world (Europe/Asia/etc) to end up at having more or less a 2:1 lead globally.
We are 3 years into this generation, which as mrcorbo put it, is what we expect to be half way. The problem with a lead such as this, is that people faced with buying either console will most likely end up with the more successful one because there isn't much that differentiates both and people tend to buy the more popular/successful one. This will continue to be the case for at least a few months until Scorpio releases. We will have to wait at least until June (or so I am reading) until Microsoft will tell us more. By then, I expect the gap between to PS4 and the Xbox to be even larger. That is a very steep battle. The only hope Microsoft (IMO) has, is to reach parity in NA and maybe surpass Sony there with help of the Scorpio. Could they do it? Certainly. NA has been a very strong market for Microsoft with the X360. But those that have already "jumped ship" and bought themselves a PS4, I can't really see them jumping ship again mid generation. Maybe some will buy both and replace one with the other eventually, but I don't expect that to be a majority.
On a worldwide basis, I expect Sony to ride this one out comfortably. The people buying 4k screens now are covered with the Pro. The only advantage I can see Microsoft has is that they are offering support for Bluray UHD (or whatever it's called) which Sony missed and might appeal to some people dishing out money for 4k screens and wanting to exploit that to the maximum by also buying some 4k movies. But they [Sony] could easily solve that by offering a newer revision later on.