The whole point of the gatekeeper designation is that they gatekeep access to things in such a way that you can’t simply take your money elsewhere. If you want to use a popular banking app, it’ll be available on exactly two platforms: iOS and Android.
We effectively have a duopoly. It's impossible for a third operating system to gain any traction and until that changes we're stuck with these two options.
A lot of things nowadays rely on apps so not having a phone with either Android or iOS means it's increasingly hard to do stuff. For example, with 2FA regulation in the EU for banks, they effectively made their mobile apps indispensable for being able to make some payments. I shouldn't have to use either iOS or Android to bank but here we are. It's even worse when you can't even use a custom ROM because the apps refuse to work on one.
