I think OP is referring to the percentage, not functionality. Windows, especially the office suites / GUIs are micht more refined. Someone somewhere pointed out at some point in time that backend development is often open source because developers are dedicated to the cause and the function. Designers, on the other hand, not so much (maybe they need payment because their main job pays less... I don't know.

In the end, the user uses the front-end, not the backend. And unless money flows into front-end development, for example, by a growing market of companies who want to switch away from office 365 for functional and financial reasons, we won't see front-ends which are attractive enough for people to switch to Linux for daily/ work related tasks.