Marks' estimates (see above) about PS4 and XB1, at least proportionally seems plausible to me, although I don't want to comment about the actual sales volume as there are too many unknown parameters to have an accurate estimate. BUT, as "Shinobi-san" indicated, MS will not let their market share slide as they have already shown this with abrupt 180 degree policy changes and free Fifa 14 bundles etc although their effect will mostly be confined to US and UK. Given the scenario (which is embedded in my estimates), the BEST that MS can hope is NEAR PARITY in US and UK (40-45%) and no matter what they do, even if they would reduce the console's price to lower than PS4, they will NEVER still sell any more than 30-40% in (pretty much) any part of the world. So this SEVERELY limits their reach globally.
You may say, "Oh well, PS4 may win the Rest of the World, but XB1 will triumph in US". That's not how it works anymore, since we are much more globalized than a couple of decades ago, and the global dominance of PS4 will have catapult effects on US as well. So MS can't play safe anymore and say "Let's have a conclusive victory at home then launch an outside attack"... If you do that (attacking unfamiliar exotic territory), you just cannot risk leaving your home defenseless, which is exactly what they are doin.
Playstation 5 vs XBox Series Market Share Estimates
Regional Analysis (only MS and Sony Consoles)
Europe => XB1 : 23-24 % vs PS4 : 76-77%
N. America => XB1 : 49-52% vs PS4 : 48-51%
Global => XB1 : 32-34% vs PS4 : 66-68%
