Making open source projects is fun. Flipping burgers and cleaning toilets isn't.

And believe it or not, people do pay for open-source programs. It's just not necessarily the end user.

People clean toilets without being paid... just not typically other people's toilets.

Have you ever looked at the breakdown of who writes the Linux kernel? It's mostly maintained by corporations, notably companies like Red Hat and strangely enough even Microsoft.

Altruism?

Many open source projects have paid contributors anyways.

Yes, among other reasons such as solving a problem for themselves or enjoyment of the process. The point is that financial motivation doesn't even explain all the production in the economy right now, so obviously removing the financial motive won't remove all production. And a basic income doesn't imply no financial motivation anyway, how many investment bankers and programmers quit their job yesterday because they already have enough savings to eat and live in a crappy sharehouse for the rest of their lives?