Killed for Skype, which was already declining by that time. Microsoft was keen on unifying their IM platforms, but failed to realise that unless the migration path is incredibly smooth, people just won't do it. And the value of any chat service is that the people you want to talk to are on there. Many people didn't bother migrating from MSN to Skype and that was the end of it.

The Skype team at the time was also run with the mindset of "developer happiness comes first, users come second", a relatively popular mindset in the 2010s, and shipped large app rewrites with missing features and usability regressions.

Of course, they eventually killed Skype too. The MSN users never went to Skype and the Skype users just progressively jumped ship to FaceTime/WhatsApp video/Google Voice to replace video calling and VoIP, respectively. By then you had a former shell of what Skype was and Microsoft figured they should just shove the remainder of their users into Teams.

Similar to the Google Talk > Hangouts > Google Chat tragedy.