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.)