That hypothetical was the case: back in 2007/2008 people of the blogosphere slowly stopped blogging and started tweeting. And if you had a good but suddenly more empty feedreader you’d then had to follow the great migration and re-find those people on Twitter.
This whole comment chain has been an interesting discussion for me, I just checked out of curiosity: Blogspot vs Twitter popularity on google trends 2004-today
It depends on the thing at hand. Some things have value because others use them. Chat apps tend towards monopolies because of this.
Email's just so damned old everyone uses the standard and it kind of dodges the issue.
Email's also structurally a bit different - it's a set of one-way sends to select recipients, with chats you moreso go to a place and read the place's signboard.
You could in principle just use mailing lists under the hood instead if the UI wasn't garbage.
