While it might seem that way to someone who is using lemmy or mastodon or any of the other fediverse apps. To a new user it will be overwhelming and almost everyone has the same first question “Do I have to create an account on all instances?” To which a lot of people familiar with the fediverse would say “No” but the caveat being you might have to create an account on more than one instance coz maybe your current instance was blocked by the other instance that you might be trying to access. It took me a day of reading exploring and understanding to get this far. So while the mail example maybe pretty simple and easy to understand it’s still not exactly as simple as that. A better example would be if you have a Twitter account you can access Gmail, YouTube, Instagram and Facebook with a single account if the other apps (instances) allow it. Also with that said there can be multiple variants of the Twitter app (instance), say Twitter 1 , Twitter 2 , Twitter 3 each with their own Communities and rules. They may or may not be able to communicate with each other depending on the moderation and community rules. So you see how even the simple examples slowly start to get complex.
A lot of Defedralization is already happening …and in most cases smaller instances will defedralize with bigger instances. That is because larger instance allow anyone to sign up so this makes it harder for smaller instances to moderate and if your arbitrary metric of 99% of users “do not give a shit” stands true then there isn’t really even a point to really join coz u are not taking advantage of what the fediverse really has to offer. The fact remains that it is complex , I am not making it complex nor is anyone on purpose trying to make it complex. Even if people were do do exacty as you say and only sign up on “lemmy.world or lemmy.ml” in most cases even that right now is not easy coz these instances are currently experiencing high traffic and are giving errors …There aren’t any good polished apps (at least on ios don’t know about android but it is a pretty similar situation) . So all and all anyone coming from the web2.0 apps will not have the easy onboarding they previously had. You are trying to oversimplify it which goes to destroy what the fediverse is trying to build that no instance is less or more important , while you dont need to sign up for every instance in existance you should still look into go explore new instances that cater to your specific needs and interests and then consider creating an account there …and since account can in most cases be migrated you can later on only have one account once you have found all the communities you wanted to join.