Google Talk was fantastic. The desktop experience was great, and the mobile apps were flawless, even on Nokia Symbian and Blackberry 6 OS.
It's the only Google product that felt fun to use. I haven't bothered with Hangouts, Allo, Duo or any of the stuff that followed it.
It didn't just die by itself. Google Talk was killed with Google+ too. It was a functional, no-nonsense IM that just worked and worked everywhere. After they engulfed this into Google+, it just became a bloated monstrosity that no one could understand. Remember when it suggested you every Google+ user when you tried to type a friend's name in Hangouts? Yeah... that was pretty fucked up.
I am just amazed that Youtube didn't die with it after it was used as a gateway to Google+.