Libre is rooted a bit in 90s design, with an OO object model designed to roughly mirror Microsoft 's COM/DCOM. I’m sure Libre has seen a lot of modernization - and I want that codebase to survive. But it’s also nice to have a second option, now.

Only Office is a Russian thing. Euro Office takes all the open source parts of Only Offices, forks them and rewrites stuff only handled by binary blobs.

It’s a quite interesting project, freeing that half-open office suite.

Yes they are. That’s why you take only the open parts of it, fork and rewrite replace all the binary blobs. That’s why you do your own change management and ignore the Russians. Read up on how the new project actually works.

Because the Only Office source is more modern while Libre Offices’s source code now is around 35 years old. At least that was the reasoning in one of the articles I read.