Well, Microsoft got themselves into a fucked up position last gen.
Because they got to #1 in America, they shifted their entire company structure to something that relied on that.
Of the three companies, they're the worst with 1st-party studios and self-made exclusives. They depend on 3rd-party sales more than any other company. AND additionally, every single major franchise of theirs is a "Me too" game that has a parallel on the PS4.
Additionally, their bread & butter profit is from Xbox Live Gold. But now with PS+ charging AND being the more popular device, it's only a matter of time until those XBL numbers dwindle significantly as people see no need to pay for both, especially as their friends are all on PS4.
So less 3rd-party sales (As most people will eventually get a PS4, and multi-plats are better there), less 1st-party games, and less XBL Gold subscribers = a LOT less sales.