Sorry for the misunderstanding. Unfortunately, I sort of feel like I would disagree more with your statement now that I understand it better? :D
I think the majority of Blender's polish came after being Open Sourced, and I don't think that movement was kickstarted by its origins. I do think its origins arguably played a role in its investment, but early Blender had a reputation of being exactly like many other Open Source projects -- powerful if you knew how to use it, but unpolished and arcane and difficult to learn.
When I first ran into the Blender, its interface was the biggest criticism I saw online about the project. Wherever the momentum came from to say "we could refactor this and make it attractive to regular modelers" I don't think that momentum was present in its early days or right after it was Open Sourced.
I'm tempted to dig into the history a bit more now, because I bet if I went back in HN history to the early 2.0 days, I would find comments saying that Blender is just like other Open Source projects in that its too confusing and difficult to use. The interface was what everyone trashed about Blender.
Of course investment helped with giving Blender the ability to undergo those kinds of radical transformations, but the transformations were sorely needed and I'm not sure that its UI was what fueled that investment. I think both overstate the connection, but I'm more sympathetic to a claim that Blender's commercial origins fueled its architecture or resources than I am to a claim that it fed its interface polish or excitement about its interface.
----
Personally, I think the biggest contributor to Blender's interface is the fact that its community is made up of a ton of artists and not just programmers. Additionally, a lot of its investment is coming from studios -- notably not from a company catered to studios, but specifically from people who are using the tool in-house as they develop it. Greasepencil in particular is heavily influenced by this; the current rewrite for Greasepencil 3.0 is being pushed and developed by people who are intimately familiar with Greasepencil as a creative tool.
In theory, Open Source should be a much better match for this than commercial software, because commercial software caters to a client-base but is developed usually by a player outside of that space that is seeking to monetize it. Open Source often falls into the same trap, but at least has the possibility to be developed and driven and to have feature requests prioritized primarily by artists and community members who rely on the software rather than by a separate entity that is interested primarily in how the software is marketed or sold. That Open Source very often fails at this is (imo) more of a commentary on the lack of community experience and community investment that most projects have. Stereotypical GUI Open Source projects are very often the result of external efforts from people who are not trained in GUI work and who are not quite as closely tied to their users.
I'd bring up Krita as another example here -- Krita has no commercial origins (https://krita.org/en/about/krita-releases-overview/) but is miles ahead of Gimp in terms of building a polished interface that caters to artists. One immediately obvious difference is that Krita's artist community is heavily engaged in the project and in regular communication with the developers and regularly gets involved in feature development and prototyping and testing. <incorrect, see correction below>In fact, Krita started out as a fork of Gimp, and yet has surpassed Gimp in terms of ease of use and polish.</incorrect>