What are considerable GNU/Linux and Open Source features not found in Microsoft and Proprietary solutions to convince my manager with my choices?

I am an open source developer at $EMPLOYER and our IT Manager plans on reducing Open Source usage to almost zero. While this is in reality impossible, he intends to use only proprietary solutions. I mean that while .NET is open source, SharePoint isn't. My tasks are mainly involve web technologies like PHP, NGINX, and MariaDB.I am not well suited to list these features since I have few time to thinker with proprietary solution and been using Open Source 100% for a too long time.What are primordial or at least good to have features that can only be found in an open source ecosystem.https://www.reddit.com/r/opensource/comments/4ufcrb/what_are_considerable_gnulinux_and_open_source/