You can run parts of an open source kernel and maybe open source user space. But forget about fully open drivers (binary blobs rule), especially remember resident platform blobs like Intel ME. The remaining exceptions are librem and some hacked thinkpads or chromebooks equipped with coreboot.
And let's not ignore the fact that most cellphones are locked up or have hardware without sane drivers, only ones provided in binary form. Not to mention the kernel running on the DSP to handle modem. Or other things.
Even higher level, just Android, try running its applications without Google proprietary services. While there is a reverse engineered version, it always plays catch up...
/e/ (e.foundation) delivers a polished google free experience that anyone can use.
so yes, it's possible.
Why is open source dead? Because mozilla layed of a bunch of people? If only their code where open source, so that everyone else could keep working on the projects they were working on. Because github makes money? And is closed source? The writing about that is particularly revealing. "if they believed open source was in principle better, they’d be open source themselves." See, there are two alternatives: Either they really believe in open source, or they see open source is something suckers do that they profit from. But in fact github open sourced plenty of important code, like the ruby git library. Thanks. Maybe it's not a dichotomy, not something that you "really believe in" or not?
It's possible today, both on desktop and on a smartphone, to run a fully open source OS. That's a huge victory by itself, and also makes those still using closed systems freer by putting a limit on what the corportaions running them can get away with.
The author dismisses any kind of open source that isn't on a sacred crusade against closed source software as corporate friendly, lacking idealism. But I think that even if there are several closed systems based on open source competing (like a lot of closed source browsers based on an open source rendering engine) that's much better then having just one closed source browser.
So yes, open source really does make people more free, partly by making it possible for dedicated people to run a fully open source system and browser, partly by increasing competition.