Having been inside the AntiVirus software industry for quite a while, and building an AV tool from the ground up, when I saw Redmond start acquiring several small AV companies in 2008 and 2009 I knew the writing was on the wall: AV will be free and part of the OS.
Refer Here for Original Post and Source https://blog.knowbe4.com/why-is-windows-defender-the-worlds-no.-1-antivirus-with-more-than-half-a-billion-endpoints