StarOffice/LibreOffice was open sourced 25 years ago. It is still maintained and developed to this day. It can run on any operating system you need, and it uses the same data formats that any "cloud solution" will use as well.
It can not be a matter of "it doesn't have online multi-writer collaboration tools", because if that was such a killer feature, there would be at least a handful of companies developing plugins to support this - or even developed in the project itself.
It can not be a matter of "data always available online", because you could solve this with a virtual online drive that can be browsed and/or synced with your work computer.
The funny thing to me is that mobile-first people prefer to have an app for each separate task, while PC-first people would rather have everything inside a browser window.