Essentially the same argument as linux-style OS vs freebsd-style OS.

Its worth pointing out the analogy that most OS have git as an addon and have emacs itself as an addon. Once someone has gone to the effort of installing git, and installing emacs, installing magit isn't a noticeable or significant barrier.

Another interesting thing for non-emacs users to consider is this saves approximately zero time. A quick

grep -c use-package .emacs.d/init.el

shows I import about 36 packages and I'm kinda a minimalist emacs user. The most effort I could possibly save would be about 3%. Its not like I'm going to stop using cider or flycheck if magit is bundled.