In what situation would that happen? If the Scarlett sells worse, then that is fewer consumers to sell to. If Xbox makes more money next gen in that scenario, it wouldn't be because of the Scarlett, it would be from them expanding their services to other platforms.
Scarlett can gain more memberships and sell less consoles than the XB1. Lets say 50m XB1 consoles were sold to the public, and only 20m were subscribers.
The Scarlett comes out and sells 40m consoles with 30m subscribers and considering Subs makes more money than Hardware (Hardware is always sold at a lost for most of its release) Also the Scarlett could sell more digital games which again profits more. Up the service subscriptions and digital output = more money especially when it adds up yearly.
You should know HW doesn't make you money, selling services and games does. Sure the more consoles sold helps sell the other two but if you market it correctly than you can find a bigger sub attach ratio as we know not all 100% of PS4 and XB1 customers actually pay for online and services.
SO if the Scarlett makes more money than the XB1, is that a fail in your books?