I think that’s a complicated decision.
First, the default format of an office suite is not just the format it saves to. It’s the format used internally by the software. Meaning that its internal classes and data structures follow the file format.
Changing the default file format is a major effort, and if you want to do that you should have just changed the office suite you decided to fork, by starting from Collabora Office instead of OnlyOffice. Which maybe they should have…
But all things considered, I can’t really fault the choice of OOXML as default. This enables them to better support all their existing documents (that’s not nothing) and also better interoperability in mixed spaces.
Even if this were to finally become the default office suite across the Union, it won’t do that overnight, and MS ans EuroOffice will have to coexist at a minimum for a loooong time.
It really is not a good look. Even though LibreOffice is still my main choice. And you have to respect the saltiness of a project that was born out of sheer spite for Larry Ellison. But it’s not a good look.