Pixelfed is bringing hundreds of thousands of users to the fediverse who don't know about federation.
I'm now overhauling our onboarding in the app so new users can login and join without the complexity of federation.
If you want to login to another server, you'll have to go into the options menu and select "Login to custom server".
Say what you will about centralization, the people are looking for a stable and ethical home for photo sharing, and I have 7 years of experience providing one.
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I think a robust solution is, as you say, to default to a large flagship instance when first running a fediverse app to ease onboarding.
But there also needs to be better account portability. Once you join a big instance and learn the lay of the land, you might want to move to another smaller instance, but if you do so, you might keep all of your followers, but none of your content will come with you.
Fix that, and users can choose their preferred instance on-demand.