I argue against your argument. Many problems are difficult to solve only the first time. For example Pythagoras hat to be quiet genius to find out the relations of the length of triangles. But one needs to be far less genius, to know and use Pythagoras rules.

The same counts for software. Crazy mathematicians have worked out the algebra behind PGP or the signal encryption. But how to send signal messages one can learn within a day or far less, even without any knowledge of algebra or mathematics.

The same way it usually is much more work needed to create functional code, then it is to detect malicious code snippets. Trust in a developer is lost within one malicious code snippet. But trust is built with thousands of lines of good code. This one way road allows FOSS to sustainably grow. Just the same as scientific knowledge is able to grow consistently based on this phenomenon.

And this reality makes FOSS sustainable. Clearly all work needs funding. Is it through a developer that self funds with other work, which is payed. Or is it through external donations.