But technically what you are saying may be right. We mostly hear about commercial work because of marketing around it. The real fun problem solving projects are often found only when you need it.
So you believe "most people" do it for the money?
There's so little money into OSS and yet so many OSS contributors, I can't see how most people could do it for money, even if they wanted to. You would have to include indirect gains to maybe become plausible that most do it for money, like support contracts, but then are they working on the OSS project to get contracts or are they getting contracts to be able to works on the OSS project.
Then the project I started decided to trigger my obsessive behaviours and take over my life. And so it goes.
We don't have "official data", this was just my opinion, based on my personal experience. I think most of the people start OSS for the reasons I mentioned, but what happens later depends on many things - success of the project, luck, type of personality of the owner, crossing paths, right place at right time, etc.
"Pure engineer" for me is someone who just wants to code, to solve a problem, and doesn't care a lot about other stuff. Even processes to organise the team (SCRUM meetings for example) are boring, waste of time. Not to mention other stuff. But that same person can, of course, change over time and go searching for other things. Me being an example - a significant part of my career I was like that, with a vision of being a technical lead or architect in the future. Until one day I realised I'm not so hyped about that anymore, that now other things motivate me. This document, 10 years ago, would be mostly new information for me. Probably not so interesting. Now it's old news for me, but it's as a "pure engineer's" good starting point, something to get him in the right direction if he wants to expand his competence toward monetising his OSS.
I think it's basically impossible to have "data" on why people work on OSS, on what their actual motivation is. All you're really going to get is anecdotal.
There are good reasons to explore ideas and participate in projects that extend beyond making pocket change through clickbait.
Not to pick on you personally, but this sort of assertion is often thrown around with no data.
It's not even clear to me that it's one or the other. For me personally it was always a mixture of both -- there's a lot of fun but I wouldn't do it without the money. But that's kinda true for my non-OSS engineering work, too!
> pure engineers
I see what you're saying, but I think there is an element of "making a product" that goes into almost any new OSS project; there is an attraction of exploring the product space from a fresh start that is part of the motivation of many many new open source projects.
(Also I think the notion that a "pure engineer" doesn't care about all those things is just wrong, but that's a whole other conversation.)