May is going to be ridiculous. Microsoft completely shot their sales momentum with that announcement.
That assumes MS has any momentum to kill. Seems they're just grasping at straws in an attempt to slow Sony's sales.
This announcement comes just days before April NPD, so I'm gonna go out on a limb and say MS already have a pretty good idea of what their non-TF-powered sales look like, especially if NPD has weeklies they distribute "privately." You're right that "Better offer coming soon!!" will lower their current sales significantly, but maybe cutting them in half won't make much difference when it comes to thousands sold. Let's say they're at 90K in April. Cutting their sales in half is bad, sure, but it's still only 45K units lost, which doesn't mean much at the end of the generation.
But, there are some number of people out there saying to themselves, "I really love XBox but I really hate Kinect, and MS say they'll never get rid of it, so I guess I'll buy a PS4 now." How many there are is hard to say, but at this point, MS are just hoping they can temporarily stem the flow of gamers to PS4. At least until E3 when they get a chance to get up on stage and show off a bunch of games that won't be out for years. If MS can get them to wait on their PS4 purchase, then there's still a
chance they'll let MS give them the bone instead.
I also think that finally taking down the paywall indicates they're preparing themselves to become an "exclusives console." PS4 is well on it's way to becoming the default console this generation, meaning that a large portion of XBone sales will come from people who already own a PS4, but still want to be able to play Halo, Forza, and Gears. Even $400 is a lot of money to spend on hardware just to play three games, and the proposition is even less attractive if the hardware in question is largely non-functional apart from the ability to play those three games. Removing the paywall makes XBone more viable as a spare console.