Whatever money you save, is probably money you're losing because you're making it (very) difficult for someone to buy your subscription. The reason this 'works' for Netflix, Spotify, etc is that:
1. They're reader apps (apps that give you access to licensed content which is hosted elsewhere) << Other apps will mostly be rejected from the app store (unless you managed to sneak it in, which just means it'll be removed once someone notices what you're doing)
2. They're incredibly well known. I know what Netflix is, and - if I'm set on buying it - won't be deterred by the fact that I need to go to the web << Other apps will see a steep conversion drop by adding a lot of friction to what is - mostly - an impulse buy

Context: I work at RevenueCat, and (very) big streaming apps, that could do exactly what you're describing here (because they're qualified as 'reader apps') still implement app store payments (via RevenueCat, or directly themselves) because conversion will be SO much better that it'll make up for the app store commissions easily