As a person who cares about the gaming ecosystem, I think it would be really healthy for Microsoft to not have full market dominance.
They're busy making studio acquisitions which are gradually centralizing the market, which could become very problematic if they start taking anticompetitive approaches to distribution.
More people on Linux means more pressure for software availability on Linux, and if people can just move over relatively easily that prevents Microsoft from going full corpo-digital-prison-hellscape.
I feel like I don't really care what my peers use, or what people in general use, but the more adoption linux desktop gets, the more people getting involved in community projects there are, as well as more bug reports and the like, so the sooner things get improved upon and the better they become.
Also more about being mainstream. I'm being forced to use proprietary centralised locked down platforms because others demand it. Free software going mainstream is one of my aims
More people using free software means more resources going into them, maybe industry-wide adoption. Would it not be awesome to have ODP be the standard file format for a document (because enough people use Linux to make an open standard necessary)? Interoperability will be a big thing if enough people start using Linux.
I have people asking me to help them install linux all the time. I am glad, in theory, but sad, in the practicality of having to work for free on my spare time.