That flexibility gives you the freedom to chose the communications platform according to your self-interests. And in today's targeted ads economy, those self-interests matter too much to neglect in favor of staying connected with everyone.
There's got to be a logical fallacy for "people did x before y technology." Yes they did. And we still collectively found out that we could do it better with that thing, and that is the new standard. But you're free to go back to delivering letters by horse if that suits you.
I can post a story on Instagram, and somebody that I have't talked to in years might reply and we have a conversation and reconnect, or maybe it's just a simple gesture, but it's still meaningful and appreciated. But if I said "well I'm here on Mastodon now and anyone who truly matters will come find me," that's just not happening. And then I'm frustrated and complain about being lonely and yell at everyone to switch to Mastodon. Really bringing people together. But at least I didn't have to stomach seeing an ad that might actually pertain to my signaled interests.
I'm pretty actively involved with my friends and just have the union of all messaging apps and even I frequently forget to respond to messages I've started to read because something else happens: baby cries etc