Linux
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It ain't perfect, but it's decent enough.
From a technical point of view, it can do anything that Microsoft Windows can. Except being forcefully installed on every non-MacOS 2019 computer you can buy.
Ciro Santilli's conversion to Linux happened around 2012, and was a central part of Ciro Santilli's Open Source Enlightenment, since it fundamentally enables the discovery and contribution to open source software. Because what awesome open source person would waste time porting their amazing projects to closed source OSes?
Ciro's modest nature can be seen as he likes to compare this event Buddha's Great Renunciation.
Linux should track glibc and POSIX command line utilities in-tree like BSD Operating System, otherwise people have no way to get the thing running in the first place without blobs or large out-of-tree scripts! Another enlightened soul who agrees.
Particularly interesting in the history of Linux is how it won out over the open competitors that were coming up in the time: MINIX (see the chat) and BSD Operating System that got legally bogged down at the critical growth moment.
You must watch this: truth Happens advertisement by Red Hat.
xkcd 619: Supported Features
. Source. This perfectly illustrates Linux development. First features that matter. Then useless features.