Even granting the rest of your argument, that math stops working for users who don’t use a particular app as their primary/only method to access reddit.

I use reddit 80% on the web, 10% on iOS and 10% on Android. I’m not going to pay reddit that full $2.50 (plus app store cut) for 10% of my usage twice. I’m just going to stop using reddit on mobile and stick to the web.

The “pay for reddit premium and get ad-free reddit plus unrestricted third-party app use” model makes more sense for users, for third party apps, and for reddit.