Users expect a company called "Mastodon" to store it, just like a company called "Twitter" stores their credentials for that site. I understand this is not how Mastodon works, so save your breath. The problem is almost everyone else on the planet does not know how Mastodon works and it will confuse and deter them.
At this point I believe more time has been spent arguing that people will be confused than people spent being confused. "Choose one, you can change it later if you want", done.
Where would you like these account credentials to be stored? Imagination land?