The big hole in your theory is that Microsoft doesn't need momentum. They already have a well established base the same as Sony. This will mostly be of interest to hardcore gamers and people with 4K TVs. It would make absolutely no sense for Microsoft to start out at $399 when this is first released. Keep in mind it's considerably more powerful than Sony's offering, which means it costs a chunk more for Microsoft to manufacture it too.
That's my guess --- $499 is a bit higher than most would like to see, but it makes the most sense for a launch price though, that way it can eventually be reduced to $449 & $399 and still be priced well above the $249 One S.
That being said, Microsoft would do way better in the long run pricing it at $399 and simply not doing any price reductions for the first 1-2 years. They'd sell way more units and gain momentum early on with the $399 pricing than they would if they sold at $499.