Earlier quoted context omitted.
I think Microsoft killed it the moment when they made everything move through central servers before everyone had good network connection, everywhere. It was P2P before. It was slow, laggy and unstable for most of the time. Also, they didn't invest in the transportation codecs much. After it's marred, they didn't try to mend it much, and when it started to work well due to better bandwidth, they didn't push it back a…
Personally I think their big, incomprehensibly stupid manoeuver was the Skype vs Skype for Business (Link) split. Had they merged them into a single client that could speak either protocol and share contact lists the story would have been very different. Why are megacorps so incomprehensibly clueless about this? Is the money pit so deep that they knock each other in while in-fighting for control on the edge of it?
It's the management decisions to try to dramatically change or replace things that lands with lesser solutions in the end. Because the incentives and bonus structure are mostly screwed up in many of these companies.
Skype could have been the best, be all, end all solution for MS and brought everyone to their knees and just killed them. But it wasn't the first, or last time this would happen.