The problem with this argument is two-fold:
1. It assumes that these experiences can’t change. They can and will, especially because much of the recent migration there is from the techie types who are currently hammering on the rough edges of the platform. Everyone forgets the sea change in Twitter usage when the first apps arrived. Remember it was web and SMS only when it started.
2. From an audience point of view, there has never been only one major platform - and, outside a narrow band of media commentary and politics, Twitter has never been that big for traffic.
Mastodon has been great at slowly capturing communities of *interest*. Any audience team would do well to keep monitoring it, to see if their audience are there in any significant numbers.