I wouldn't be surprised at all if Scorpio is $399, in fact, that's what I'm expecting. MS didn't build this thing so that it wouldn't sell, they're trying to regain traction amongst the hardcore gamers that left them en masse at the beginning of the gen. Pro isn't going to cost $399 this fall, either, I would bet. Scorpio is built with similar parts as the Pro, even, with a custom polaris gpu and custom jaguar cpu, albeit more (and more expensive) Ram and a likely more expensive cooling system to get clock speeds higher. So it will cost more than Pro, but considering Pro is likely to drop price this year, a $399 price point will likely be attainable.
And yes, a console coming out in 2019 or 2020 with less concern about CPU compatability will be able to be quite a bit more powerful with a Zen + Vega combo (or maybe even a better GPU, considering the timeframe). I'd bet MS will have a similarly more impressive system of their own to release at that time as well.
