One reason it should bother people that #Mastodon is a singular gateway is because so many may dislike the Mastodon experience and never go farther, never see other things that #Fediverse can provide.
It's like counting on some barely edible chain restaurant food to introduce people to an otherwise wonderful cuisine.
(You can insert your own example there :) )
People turned off by Mastodon's operation and interface may never sign up for other instances that would serve them better.