The reason Apple doesn't allow any other browser engines on iOS is due to them collecting up to 30% of purchases made through the apps from the app store. If a developer can do the same things with a capable web browser, then they won't need to create a native iOS app and that cuts into Apple's app revenue. So Apple purposely hobbles Safari so it doesn't have any advanced browser APIs for stuff like bluetooth or other APIs that apps have access to, forcing developers to create an app, where Apple can then cut into purchases made through the app.

It has nothing to do with people no longer using Safari and Apple being sad about that. Other browsers can technically be installed on iOS, but the underlying browser engine is forced to be Safari, which lacks many APIs other web browsers could implement, reducing the need for a native app. It's purely Apple's anti-competitive greed that drives this situation. And the EU, Japan, and the US DOJ have noticed. So far only the EU and Japan have actually taken measures to force Apple to change this.

Here's the entire DOJ lawsuit which includes many other instances of anti-competitive practices by Apple.

https://www.justice.gov/archives/opa/media/1344546/dl?inline