10% of users with MacOS Office 2019 installs just got NERF'd by Microsoft. This story will not encourage users to spend more money on a disappearing rabbit trick. =3
It does not even have to be in a web UI or browser. Just somehow make it possible to easily connect and edit collaboratively. I know, I know, it is a huge ask, unfortunately.
LibreOffice Writer online was not popular, so it is understandable you assume it doesn't exist. There is also the trivial headless Linux remote desktop cloud hosting with 100% identical functionality.
>"edit at the same time"
Perhaps you meant: "View -> Toolbars -> Track Changes..."
Most documents that support OLE have difficulty handling concurrent writes. Office 365 abandoned desktop publishing in 2019, and replaced it with a web document management system.
Best workaround is every user imports each instanced changes: "File -> Merge Document."
And manually handle any collisions.
I would post how to setup your own remote collaborative environment on an $8/month host, but people seem like they are not interested. =3