It's possible on the Mac, but it's not easy. Apple uses an immutable system volume on macOS, so you can't just delete the Safari app like you would a user-installed app. To actually delete Safari you need to disable System Integrity Protection and reboot.

There are plenty of Linux distributions that use immutable root volumes. They protect the user in a huge number of ways by preventing the system from getting hosed (either by accident or by malicious unauthorized users / malware). Apple made the decision to do this for their users, and it has prevented a HUGE amount of tech support calls, as well as led to millions of happy users with trouble-free computers.

It also hasn't stopped users from installing Chrome and/or Firefox on their Macs, and millions of ordinary users have.

> It also hasn't stopped users from installing Chrome and/or Firefox on their Macs, and millions of ordinary users have.

You seem to be ignoring the part where you can't install the Chome and/or Firefox browser engines on iOS and the apps with those names on that platform are just skins over Safari. Notice in particular that the iOS version of "Firefox" can't support extensions.