I don’t know about radical; if you’re talking about the same sorts of Linux environments I’m thinking of (e.g. a desktop running GNOME, using FOSS GUI apps), then, ignoring the kernel itself [which is a special case in many ways], their developers are—by volume of commits—mostly community contributors.
Would you expect that said community contributors would have any contribution/engagement at all, if the product they were contributing to wasn’t 1. sitting there with its source available on GitHub, in a state where you can compile and run your own copy of it from said source, and 2. accepting PRs from all comers, with project direction mostly up to either “the guy who wrote the code originally”, or “the whims of the people who do the most PRs”, such that 3. you gain in decision-making power in the project to the degree you contribute?
Once you have those three properties, you’ve essentially got FOSS. It doesn’t really matter if you charge for the binary distro (like e.g. RedHat does) if the developers behind the source code are essentially their own little monastery/guild, run by the will of the people who write the code, that can just up and strike if the product’s corporate owner does something stupid. If it isn’t FOSS, it’ll be forked or re-built from the ground up by said developers to be FOSS; not out of principle, but in order to put “the people writing the code” in power over the code project’s direction. (See e.g. LibreOffice.)
I wouldn’t say that making a specifically-FOSS piece of software was ever a “goal” of any of these projects, outside of Stallman’s GNU itself; rather, these projects being FOSS is just a sort of emergent property from the “anthropic principle of community-driven software engineering”: for software to exist, people—a lot of people—have to have had a hand in making it; and, unless they all worked for the same company, there has to have been some legal infrastructure in place to make many different parties able to push the project forward. That legal infrastructure is a license like the GPL or BSD. The projects that had that infrastructure thrived; the projects that didn’t have it—and there are many—stagnated or died.
(This effect only applies above a certain level of complexity, mind you; there are small software packages that are “free” but where nobody but their author has ever contributed to them. Compression libraries like zlib, and format libraries like libpng, tend to be like this; as do smaller library-ecosystem packages like your average NPM package is. In those cases, there isn’t much to grow, so stagnation is just fine. But, don’t be confused, stagnating still is what those packages are doing.)