I think people should get over with "file formats"
Cloud/browser based office tools were already a thing decade ago. Do we have an open format except .md ? Sadly no.
Afterall, format is just a way to serialize features.
So lets have all your data vendor locked in a cloud somewhere so that the corp runnning it can milk you for ever or just decide to delete it all on a whim ? Perfect!
And what if those "cloud based office tools", which would be using proprietary file formats internally, shut down? Or heck, what if they announce to raise their pricing? All of your docs gone. No possibility to migrate.
Cloud based tools are even worse than proprietary file formats for lock in.