They could only do so if they knew the majority of their customers would follow them, despite the significant friction (and hence attrition) this would entail vs. using the default App Store. It would be unthinkable to most developers. The app would need to be e.g. an insanely popular game to
only be available in an alternate store.
Plus, the alt store would need to offer hugely favourable terms compared to Apple to make the attrition worth it. Which raises the question of how much profiteering is in Apple's offer. In practice, Apple would likely just reduce their take a bit.
A key benefit of alt stores is in keeping Apple honest. It weakens their control of the platform, to the customer's advantage, because they don't get the final say on everything - they're forced to earn their customer's favour.
So do I, but they're unlikely to bother with alt stores anyway. They're perfectly capable of using Macs / PCs without getting infested with viruses, and that's without iOS's strict sandboxing. So I'm sure they'll be fine.
Then don't enter your credit card details in any alt stores you don't approve of. Companies like Epic already offer detailed parental controls, that go way further than just approving purchases -
https://safety.epicgames.com/en-US/parental-controls.