Sure it would give Mastadon the power to pick and choose who it wants to host, but if you want to go join a different instance you are more than welcome to.
It would just substantially streamline the process for your mainstream user.
Relays serve this purpose. I use one on my personal one-user instance.
https://joinfediverse.wiki/index.php?title=Fediverse_relays&...
Worked as an interesting relay solution to my small instance. I relayed most of the instances which contain profiles I've subscribed to and it filled out my timeline considerably. Replies to remote posts aren't downloaded by default and this solved that problem.
I mean, isn't any instance in the federation already offering a proxied cache of the content of all other instances you access through it?
You can just register/login in mastodon.social website the same way you can register/login in Twitter, and then access cached copies of content from the entire fediverse compatible with Mastodon.
If you are gonna make new accounts connect through a global mainstream proxy, you might as well just let them connect through a global mainstream instance. I personally do not see the problem with one instance being more mainstream than the rest as long as federation is still respected. It's definitelly still an improvement over centralization and if it ever becomes a burden it'd be easier to migrate.
Mastodon? It's a dictionary word, so it as that going for it—and yet, people see it a hundred times and still often misspell it. The pronunciation's just a bit too far from the spelling, at least in American English. Most folks say it as if it were spelt "mastadon", especially if they're not trying to enunciate carefully & precisely and (perhaps) don't know the correct spelling.
(I'm not weighing in on whether that's some kind of serious problem for Mastodon, but I do think it's worse than all your other examples, as far as likelihood of misspelling even after significant exposure—then again, web search smooths over most of that, for anything sufficiently popular)
You see the same thing with other words that have "o"s that function similarly. You'll hear Americans turning "o" into "uh" in Tyrann-o-saurus, p-o-tato, etc.
Especially if we're talking about an Internet thing: you've seen it in written form more often than you have heard it enunciated out loud. And seems like the iOS text prediction is also able to spell it the right way.
I don't care honestly, but as a non native English speaker I've never understood why natives make an enormous amount of banal spelling errors.