Even if all of the operating systems were playing on a fair & ideal field, I do not think Linux would come out as the clear winner.

The Linux ecosystem is stakeholder owned. That is to say that design decisions are made by experienced users for experienced users. Whenever an ergonomic tradeoff exists between ease of use and expressiveness, ease of use loses. New users sense this and feel implicitly unwelcome. It's the original sin of open source software as a whole, really.

I don't necessarily take this state of affairs as a bad thing, but it does lead me to think that the dominant OS software will always be a commercial product of some variety. It doesn't necessarily need to be a proprietary greed-fest like Windows, but at the very least the top-level stakeholders of that specific project need to be directly motivated by user adoption. AOSP (aka: Android) would be a decent example of something like this working in the wild for an open source project (Google attempting to claw back control notwithstanding).