I'm not saying they're not related, just not linearly so. For one, screen time is scalar and subscription is binary. My screen time only needs to be high enough

I'm not saying they're not related, just not linearly so. For one, screen time is scalar and subscription is binary. My screen time only needs to be high enough for subscription binary to remain active across time. It's possible that screen time is simply the most highly correlated and easiest to track metric they use, which is likely. Another possibility is that they do recognize that screen time probably has logarithmic falloff as it correlates to subscriptions, Fortnite is eating enough of the screen time away that it's in the range that is highly likely to influence whether or not they keep their subscription.