People should realize when what they write is public though. The world retreating into private group chats is not a bad thing.
but people like these researchers who only want to exploit are making it necessary
1. The illusion of intimacy
2. The illusion of ephemerality
People think they chat to a small number of people, and people think that it's going to go away at some point.
So they think they're in a private conversation when they're not. They wouldn't behave the same if they realized what they say is being written down and stored forever in some database.
And yes, it sucks that technology doesn't allow you to have it both ways => public because you want to reach far and wide, and private cause you don't want it recorded.
Look at it like this: There's no chance in hell that intelligence agencies, hacker groups, and whatever other nasties you care to worry about haven't already been using archives just like this for all their nefarious purposes. They just didn't make their usage public because why break the honey pot?
What these researchers did is show what was possible and make their efforts public. Now you are better informed of what was always possible. It's always been necessary to think carefully before putting stuff on the internet, it's just now your bubble is burst.