Alienate the Xbox user base? Every Xbox community I've visited, Scorpio is the main conversation, nobody is upset they bought an S model with Scorpio coming out. Everybody is super stoked to see Scorpio, find out what it looks like, how much it costs, when they'll be able to buy one, etc.
The entire Xbox community is dying for Scorpio, even those that won't be purchasing it for a year or two. If they don't lead with the Scorpio reveal, nobody is going to be paying attention to what is going on because that would be the elephant in the room. I think roboto has it perfectly right. They show off Scorpio, launch date, price, and then they show off the games. Actually, knowing MS, they'll probably lead the conference with Bleem or Belch or whatever their streaming service is, won't mention the Scorpio at all, and spend the rest of the time making jokes nobody thinks is funny.