I suppose that until you find an open source operating system that doesn’t work, this is a hypothetical and/or philosophical discussion.
Let us know how Guix goes.
Purism certainly doesn’t try to stop you installing whatever software you want e.g. no locked bootloaders etc. - and since Purism hardware is as open source-friendly as you can get, with enough effort, you should be able to make any open source operating system work.
However I don’t think that anyone can guarantee that just any open source operating system will work or provide support for just any open source operating system.
As PureOS is in the Debian family, something else in the Debian family is the least change and most likely to work.
Then you can step outside the Debian family to other non-Debian Linux distros.
Then you can step outside the Linux family to other Unix-related distros (e.g. BSD-related).
Then you can step outside the whole Unix universe - and, well, who knows. Your mileage may vary.
I don’t speak for Purism. These are just my opinions.