^I watched it live and she was okay for the first few seconds of Like A Prayer but then she started singing out of tune and it stayed that way. She also souded very out of breath from just walking down the stairs. Honestly, I can get why she got so much backlash since her live vocals weren't good and the performance's concept didn't save much. She was paid a lot too apparently and she got hyped up the whole night. I do think it is exaggerated to say that it ended her career like, yes it was bad but who knows what went wrong. It can happen but she simply did not deliver (vocally and performance wise) so people reacted a certain way. She just sounded very weak and if it's age then oh well.. If it was something else then one can only hope that future performances will sound better.
The masks were quite tasteless in my opinion, such as the whole political statement thing. The ESC didn't allow the contestants to make such since it's supposed to be more about the songs and fun. But then again, that's Madonna.
They edited/used a different audio for the performance on YouTube (it has some obvious autotune) and I honestly don't have a problem with that but after 200 million people hearing the live vocals, it wouldn't surprise me if this is the next thing they laugh at.
Edit: they delete many other videos of the un-edited performance but here you can check parts of Like A Prayer.
https://youtu.be/Arq5XQXuI98
https://youtu.be/FVyyZZV0UPk