These researchers are just using the tools that Discord made available to archive information that was already public. If anyone is to blame for users believing that their messages were private or ephemeral it's Discord, not the researchers.

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.