So if "[e]ven when there is a permissive license, it's still incredibly difficult for a new file format to gain any traction" then there there would seem to be nothing lost by licensing the reference implementation under the GNU GPLv3 despite your vague claim that the GPL hinders "broad adoption". You say "If the ultimate goal is to promote this file format, this is not the best way of doing it" but you say nothing about what "the best way" is or what constitutes "best".
"FOSS" means free and open source software, software released under a license approved of by both the Free Software Foundation and the Open Source Initiative; there are many other such licenses. It's unclear what you mean by "FOSS/GPL" as being somehow distinct from the "GPL" (meaning the GNU General Public License), a term used for decades which requires no qualification. Perhaps you're confused by the term "GNU/Linux" which is the GNU operating system in combination with the Linux kernel (as opposed to, GNU/HURD or GNU/kFreeBSD, to name a couple of examples, which are the GNU OS with the HURD or FreeBSD kernels respectively).
For being moderated as insightful I see a self-contraction, unclear use of confused terminology, and a complaint hinting that something far better should be done without any explanation of what that is.