Let's assume for a moment that mastodon is growing still. If Google adds it to everybody with a Gmail account, then hardly anybody will sign up for a non-Google server. It's easy to imagine a world in which most people are accessing via Google's servers.
And RSS was fine before Google Reader came around.
And was fine afterwards.
it is still fine
Google Reader is actually a good example. RSS didn’t die, but the Google Reader death pushed a lot of people into Twitter for reading news.