Linux

Didn't see the original analysis that is used to make that claim, so I can only argue about more general problems, specifically to what scopes one can keep using only software he/she can audity and is free to use. For example, work may require tunelling tools or email/messaging programs which you have no control over. Also the majority of games have at least closed source elements. You also can't guarantee the sites you access are FOSS or the hardware you have available makes its blueprints or firmwares available and auditable. Also if your setup grows, e.g. using a game controller, USB stick, another screen, etc., drivers needed and/or installed could be a "black box" too as you say.

Such an end is still utopic, too far to reach, so my suggestion would be trimming what can be replaced or ignored while rising awareness (and possible solutions¹) to specific problem, and through that step by step move closer to such a fully FOSS objective.

¹pointing problems without proposing solutions, that I can observe, is a great way to make the listener lose his/her will to tackle a given problem