From their position of power they don't need anymore to prevent users from installing other browsers; it's enough for them to continuously pester the user with

From their position of power they don't need anymore to prevent users from installing other browsers; it's enough for them to continuously pester the user with annoying requests to make their own the default one until the user surrenders. For 99.999% of users, a browser that doesn't load by default and a browser that is not installed are essentially the same thing.