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